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通过预编译头文件的方法来提高c++builder的编译速度(英文原版) (转)

原创 IT综合 作者:worldblog 时间:2007-12-03 16:15:51 0 删除 编辑
通过预编译头文件的方法来提高c++builder的编译速度(英文原版) (转)[@more@]
Improving C++Builder Build Times With Pre-Compiled Headers.
C++Builder is one of the fastest C++ compilers around, and probably the fastest win32 C++ compiler in terms of compilation speed. Despite the speed advantage that C++Builder holds over other C++ compilers, many Delphi programmers contort in agony while waiting for a C++Builder project to compile. Anyone that has seen Delphi knows that it is blazing fast in comparison to any C++ compiler. Delphi can compile small example projects in less than a second, and large projects can be built in less than five.

So why does Delphi hold such a speed advantage over C++Builder? Furthermore, is there anything that can be done to improve the compilation speed of C++Builder? This article explains why C++ compilers are inherently slow, and demonstrates a simple tactic to boost compile times in C++Builder.

Why C++ Compilers are Slow
How C++Builder Uses Pre-Compiled Headers To Reduce Compile Times
Explanation of Pre-Compiled Headers in a VCL GUI Project
Optimizing C++Builder's Use of Pre-Compiled Headers
Results
Notes



Why C++ Compilers are Slow
In C++, you cannot call function from a source file unless that function has been previously defined or declared. So what does this mean? Consider a simple example where function A() calls function B(). A() cannot call B() unless a prototype for B(), or the function body for B(), resides somewhere above the function body for A(). The code below illustrates this point.:

// declaration or prototype for B
void B();

void A()
{
    B();
}

// definition, or function body of B
void B()
{
    cout << "hello";
}

The code will not compile without the prototype for B(), unless the function body for B() is moved up above A().

Function prototypes serve a crucial role to the compiler. Every time you execute a routine, the compiler must insert proper code to call the routine. The compiler must know how many parameters to pass the function. The compiler must know if the function expects its parameters on the stack or in registers. In short, the compiler needs to know how to generate the correct code to call the function, and it can only do this if it has seen a previous declaration or definition for the function that is being called.

To simplify the prototyping of functions and classes, C++ supports a #include statement. The #include directive allows a source file to read function prototypes from a header file prior to the location in code where the prototyped functions are called. The #include directive plays an important role in Win32 C++ development. Function prototypes for C RTL functions are provided in a standard set of header files. The Win32 api is prototyped in a set of header files provided by microsoft, and the classes and functions of the VCL are listed in header files that come with C++Builder. You can't create a very useful windows program without including header files provided by Microsoft or Inprise.

Header files help implement C++ type checking in a manner that is easy to manage for the programmer. However, this benefit comes at a huge cost. When the compiler runs across a #include directive, it literally opens the included file and inserts it into the current file. The compiler then parses the included file as if it was part of the file that it was already compiling. So what happens if the first header file includes yet another file? The compiler will suck in that file and start parsing it. Imagine what happens when 10, 20, or even 100 files are included? While this number of include files may sound large, it isn't unrealistic when you start adding up the Windows SDK header files and all of the VCL header files.

To demonstrate how the compiler branches off and translates included files, consider the following code example. This is a simple console mode program that I built using the Console Wizard from the object Repository. In order to test this code, select Options-Project-Compiler and turn off pre-compiled headers.

// include some standard header files
#include
#include
#include
#include

#pragma hdrstop
#include

//-----------------------------------------------
int main()
{
    printf("Hello from printf.n");
    cout << "Hello from cout" << endl;
    MessageBeep(0);
    return 0;
}

When I build this project with C++Builder, the build progress dialog reports that the project contains 130,000 lines of code. 130 thousand lines! How can that be? The source file only contains about four lines of code. The 130,000 lines were contained in STDIO.H, STRING.H, IOSTREAM.H, WINDOWS.H and all of the other header files that are included by these four header files. In this example, the compiler spent the vast majority of its time processing header files.

Now let's investigate what happens when you have multiple CPP files in a project. Building off of the existing project, let's add a unit to the console program that we already have. Add a simple function to this second unit. Then alter the first CPP file so it will call the new function.

//-------------------------------------------------------
// UNIT1.CPP
#include
#include
#include
#include
#include "Unit1.h"        // prototype A() in unit1.h

#pragma hdrstop

void A()
{
    printf("Hello from function A.n");
}
//-------------------------------------------------------

//-------------------------------------------------------
// PROJECT1.cpp
#include
#include
#include
#include
#include "Unit1.h"

#pragma hdrstop
#include

//-------------------------------------------------------
USEUNIT("Unit1.cpp");
//-------------------------------------------------------
int main()
{
    printf("Hello from printf.n");
    cout << "Hello from cout" << endl;
    A();
    MessageBeep(0);
    return 0;
}
//-------------------------------------------------------

Now build the project. If you turned off pre-compiled headers before building the project, you will see that the compiler progress dialog reports 260,000 lines of code when the build finishes. Notice that the compiler had to translate the same set of header files in two different CPP files. We know from the previous example that these header files place a burden of 130,000 lines on the compiler. The second CPP file places the same 130,000 line burden on the compiler, for a grand total of 260,000 lines of header files. Imagine how this line count would rapidly grow in a large project. The burden of processing the same group of header files over and over can greatly increase compile times.



How C++Builder Uses Pre-Compiled Headers To Reduce Compile Times
The engineers at Borland realized that they could decrease build times by designing a compiler that did not process the same header files over and over during the build. To achieve this goal, Borland C++ 3.0 introduced the concept of pre-compiled headers. The idea behind pre-compiled headers is relatively simple. When the compiler processes a set of header files for one particular source file, it saves the compiled image of the header files on the hard drive. When that set of header files is required by another source file, the compiler loads the compiled image instead of processing the header files a second time.

Let's modify our console mode program to see how pre-compiled headers can impact build times. The code itself should already be fine. We just need to turn the project's pre-compiled headers option back on. Select Options-Project-Compiler and check the Use pre-compiled headers option or the Cache pre-compiled headers option. Enter PCH.CSM in the pre-compiled header filename box. Do a full rebuild of the project once you change the settings.

During the build, pay special attention to the compiler progress dialog. You should see that the compiler processes 130,000 lines of code when it compiles PROJECT1.CPP, but when it compiles UNIT1.CPP, it only process 20 lines of code. The compiler generates a pre-compiled image when it parses the first source file, and that pre-compiled image is used to speed up compilation of the second source file. Imagine the performance boost that you would attain if the project contain 50 source files instead of 2.



Explanation of Pre-Compiled Headers in a VCL GUI Project
The use of pre-compiled headers in the previous example reduced the build time of the project by almost 50%. But that was a simple console mode program that didn't do much. You probably want to know how you can take advantage of pre-compiled headers in a full blown VCL GUI program. By default, C++Builder automatically turns on pre-compiled headers for you. However, C++Builder does not pre-compile every header file that is used by your program. It only pre-compiles the file VCL.H, which you can see by inspecting the top of any form's source file:

#include
#pragma hdrstop

The #pragma hdrstop directive tells the compiler to stop generating the pre-compiled image. Any #include statement located before the hdrstop directive will be pre-compiled, while any #include below the directive will not be pre-compiled.

So how many header files get pre-compiled when VCL.H is pre-compiled? If you look at VCL.H, you will see that it includes another file called VCL0.H (assuming you have BCB3). If you don't alter the default settings of C++Builder, VCL0.H will include a small set of VCL header files. They are:

// Core (minimal) VCL headers
//
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include

This is a small cross section of header files, and it probably represents only a subset of the header files that are used in a moderate to large sized project. VCL0.H does allow you to pre-compile more header files through the use of conditional defines. You can #define a variable called INC_VCLdb_HEADERS to pre-compile the VCL database header files. Likewise, you can define INC_VCLEXT_HEADERS to pre-compile header files for the extra controls that come with C++Builder. If you define a variable called INC_OLE_HEADERS, C++Builder will pre-compile some of the SDK com header files. These defines should be placed before the #include statement for VCL.H.

#define  INC_VCLDB_HEADERS
#define  INC_VCLEXT_HEADERS
#include
#pragma hdrstop

Note: If you decide to try this technique, make sure you add to the two defines to every CPP file, even if they don't use DB classes or extra controls. The reasoning for this will be explained shortly.



Optimizing C++Builder's Use of Pre-Compiled Headers
The default pre-compiled header settings do reduce the time it takes to build a project. You can prove this fact by timing a full build of a large project when pre-compiled headers are on and by timing the build when pre-compiled headers are off. The goal of this article is to tweak the way C++Builder pre-compiles files to reduce build times even more. In this section, I have outlined two techniques for improving build times.

Before we discuss the techniques, it important to realize how C++Builder decides that it can use an existing pre-compiled image when compiling a source file. C++Builder generates a unique pre-compiled image for every source file in your project. These pre-compiled images are saved in a file on your hard drive. The compiler will re-use an existing pre-compiled image when two source files require the same pre-compiled image. This is an important distinction. Two source files will require the same pre-compiled image if they include exactly the same files. Furthermore, they must include the files in the same order. Simply put, the source files must be identical up until the #pragma hdrstop directive. Here are some examples:

    Example 1: Pre-compiled images don't match
    //--------------------                  //--------------------
    // UNIT1.CPP                            // UNIT2.CPP
    #include                       #include
    #pragma hdrstop                        #pragma hdrstop


    Example 2: Pre-compiled images don't match
    //--------------------                  //--------------------
    // UNIT1.CPP                            // UNIT2.CPP
    #include                       #include
    #include                   #pragma hdrstop
    #pragma hdrstop


    Example 3: Pre-compiled images don't match
    //--------------------                  //--------------------
    // UNIT1.CPP                            // UNIT2.CPP
    #include                       #pragma hdrstop 
    #pragma hdrstop                        #include


    Example 4: Pre-compiled images match
    //--------------------                  //--------------------
    // UNIT1.CPP                            // UNIT2.CPP
    #include                       #include
    #include                     #include
    #include                   #include
    #include                     #include
    #include "unit1.h"                      #include "unit1.h"
    #pragma hdrstop                        #pragma hdrstop

    Example 5: Pre-compiled images match
    //--------------------                  //--------------------
    // UNIT1.CPP                            // UNIT2.CPP
    #define  INC_VCLDB_HEADERS              #define  INC_VCLDB_HEADERS
    #define  INC_VCLEXT_HEADERS            #define  INC_VCLEXT_HEADERS
    #include                         #include
    #pragma hdrstop                        #pragma hdrstop

    #include "unit1.h"                      #include "unit2.h"


    Example 6: Pre-compiled images don't match
    //--------------------                  //--------------------
    // UNIT1.CPP                            // UNIT2.CPP
    #define  INC_VCLDB_HEADERS              #include
    #define  INC_VCLEXT_HEADERS            #pragma hdrstop
    #include
    #pragma hdrstop

When the compiler processes a source file with a pre-compiled image that does not match an existing image, the compiler will produce a completely new image from scratch. Look at Example 2 above. Even though STDIO.H is compiled along with UNIT1.CPP, the compiler will translate STDIO.H again when it compiles UNIT2.CPP. Pre-compiled headers reduce compile times only when the compiler can re-use an existing pre-compiled image across multiple source files.

This is the foundation for both of the techniques that I list here. Pre-compile as many header files as you can, and make sure that you use the same pre-compiled image in every source file.

Technique 1:
The first technique is to simply boost the number of files that VCL.H includes by adding two conditional defines to every source file. Open every CPP file in the project, including the project source file, and change the first two lines of the file so they look like:

#define  INC_VCLDB_HEADERS
#define  INC_VCLEXT_HEADERS
#include
#pragma hdrstop

If you don't like the idea of adding these defines to every source file, you can accomplish the same thing by adding INC_VCLDB_HEADERS and INC_VCLEXT_HEADERS to the conditional defines line under Project - Options - Directories/Conditional.

You might want to throw in some of the C RTL header files that you commonly use, along with WINDOWS.H. Make sure that you add the lines before the hdrstop pragma, and make sure that you list them in the same order in every C++ source file.

#define  INC_VCLDB_HEADERS
#define  INC_VCLEXT_HEADERS
#include
#include
#include
#pragma hdrstop

Technique 2:
Technique 1 works fairly well, but it isn't very flexible. If you decide to add a new header file to the list of files that get pre-compiled, you need to modify every C++ source file in your project. Furthermore, Technique 1 is prone to error. If you mess up the order of your includes, you can actually make your compile times worse, not better.

Technique 2 addresses some of the downfalls of Technique 1. The strategy here is to create one huge header file that includes every header file that is used in your project. This single file will include the VCL files, windows SDK header files, and RTL header files. It can also include all of the header files for forms and units that you have created, but as we will see later on, you don't want to pre-compile header files that are likely to change (see the note entitled Don't pre-compile header files that change).

Here is an example of what the common header file will look like:

//---------------------------------------------------------
// PCH.H: Common header file
#ifndef PCH_H
#define PCH_H

// include every VCL header that we use
// could include vcl.h instead
#include
#include
#include
#include
#include
#include
#include
#include

// include the C RTL headers that we use
#include
#include
#include

// include headers for the 3rd party controls
// TurboPower System
#include "StBase.hpp"
#include "StVInfo.hpp"

// Our custom controls
#include "DBDatePicker.h"
#include "DBRuleCombo.h"
#include "DBPhonePanel.h"

// Object Repository header files
#include "BaseData.h"
#include "BASEDLG.h"

// project include files
// pre-compile these only if PRECOMPILE_ALL is defined
#ifdef PRECOMPILE_ALL
#include "About.h"
#include "mainform.h"
...
... // about 60 more files
...
#include "validate.h"
#endif

#endif

Once you have the gigantic common header file ready, change every source file so it includes this file. I have chosen to leave the original include statement for VCL.H intact. You might want to move VCL.H to the common header file.

//-----------------------------------------------
#include
#include "pch.h"
#pragma hdrstop

Note: After you add the include for PCH.H to every C++ source file, don't insert any more include files prior to the #pragma hdrstop. Doing so will cause those C++ files to require a pre-compiled image that does not match the pre-compiled image from other files.


Results:
I am currently employing Technique 2 without defining PRECOMPILE_ALL on my current project. The project is a medium sized client/server database program that consists of 113 C++ source files, most of which are forms or datamodules. Using Technique 2, a full build of the project takes only 195 seconds. Of that 195 seconds, 40 seconds are spent generating the pre-compiled header, and about 40 seconds are spent linking. In the remaining time, the compiler translates 113 C++ source files. That's an average of one file per second. By way of comparison, the project takes more than 30 minutes to build when no pre-compiled headers are used, and the project takes 18 minutes to build when pre-compiled headers are used but Technique 2 is not utilized.

Incremental makes with Technique 2 are lightning fast when no header files have changed. The compiler does not bother to regenerate the pre-compiled image on disk if no header files have changed. When this condition is met, an incremental make takes only 1 or 2 seconds, because only the C++ source files that have changed need to be compiled. The compiler spends all of its time compiling those files, instead of wasting its time compiling system header files. When a header file does change, the speed of an incremental make depends on whether or not the PRECOMPILE_ALL flag was defined.


Notes:
Don't pre-compile constant variables: The compiler cannot pre-compile a header file if it contains a constant variable that is assigned a value. For example, placing the following line in a header file can interfere with the creation of a pre-compiled header image:

const AnsiString strError = "An Error Occurred!!!!!";

If you want to place const variables in a header file, create a separate header file to contain the constants and don't pre-compile that file. Try to reduce the burden on the compiler by not allowing this header file to include other files. Similarly, don't include this file from other header files if you can help it. If this seems like a difficult task, you can use the extern keyword. Create a header file that contains extern prototypes for your constants. Then create a CPP file that defines the constants (ie gives them a value).

Note that the problem of const variables only occurs when you do define the PRECOMPILE_ALL flag. When you do not define this flag, your own header files are not pre-compiled. If you don't pre-compile a header file, then you can add constants to it without any problems. Also, #define's do not present a problem, just const variables (although I don't recommend that you switch back to #define's).

Don't pre-compile template headers: This suggestion is based on empirical evidence. I have a template class with several inline functions. The entire template class resides in a header file. The compiler was able to pre-compile this header file, but I noticed that the pre-compiled image was always re-generated during an incremental make. I think this has to do with the way templates are handled by the compiler. I can pre-compile the STL headers without any problems, so I'm not sure what the problem is. I suggest that you go ahead and try to compile template headers, but pay close attention to the compiler progress dialog. You may need to stop pre-compiling template headers if they cause problems.

Keep an eye on the compiler progress dialog: The compiler progress dialog tells you how well your pre-compiled headers are working. When you employ Technique 2, you should see that the compiler takes a long time to compile the first C++ source file in your project. The compiler generates the pre-compiled header image during compilation of the first file in the project. During this time, you should see the line count on the compiler progress dialog reach a huge number (100,000-500,000). Once the compiler moves on to other C++ files, the line count should probably be between 20 and 1000 lines for each source file if you define the PRECOMPILE_ALL flag. If you don't define this flag, the line count should stay under 15000 or so. Once the compiler finishes translating the first file in the project, subsequent files should only take a second or two to compile.

If the compiler gets bogged down on one C++ file for more than 4 seconds, you probably have a source file whose pre-compiled image doesn't match the image created by the common header file. The line count is another indicator. If you see the line count sail up above 50,000 lines for one source file, it's a good indication that the compiler was unable to apply the existing pre-compiled image to that source file.

Don't pre-compile header files that change: When using Technique 2, realize that any small change to a header file will force the compiler to regenerate the pre-compiled image. Based on the test results, this could take from 20 seconds to a minute. If your header files change frequently, you may want pre-compile only system and VCL header files. This is the purpose of the PRECOMPILE_ALL flag. It allows you to easily include or remove your header files from the pre-compiled image.

//---------------------------------------------------------
// PCH.H: Common header file
#ifndef PCH_H
#define PCH_H

// include every VCL header that we use
#include
#include

// include the C RTL headers that we use
#include
#include
#include

// project include files
// pre-compile these only if PRECOMPILE_ALL is defined
#ifdef PRECOMPILE_ALL
#include "About.h"
#include "mainform.h"
...
... // about 60 more files
...
#include "validate.h"
#endif

#endif

To pre-compile your own header files, add PRECOMPILE_ALL to the conditional defines line under Project - Options - Directories/Conditional. If your header files change frequently, then don't add this conditional define. When you don't pre-compile your own header files, a full build of your project will take a little longer. However, when you make a change to one of your header files, an incremental make will be faster because the compiler won't waste 20-60 seconds rebuilding the pre-compiled image.

I do not define PRECOMPILE_ALL for the project that I described in the Results section because I found I was still changing my header files frequently, and incremental makes were taking more than 2 minutes. The table below illustrates how the PRECOMPILE_ALL directive affects compile time. I timed how long a full build took when the PRECOMPILE_ALL was defined. Then I made a small change to the header file for my main foRM and performed an incremental make. Next, I repeated this process with the PRECOMPILE_ALL value not defined. Here are the results.

Not defined (do not pre-compile my headers)
-------------------------------------------------------------
Full Build: 195 sec 408887 lines compiled
Inc Make  :  28 sec 7 files affected: 27059 lines compiled

Defined    (pre-compile my headers)
-------------------------------------------------------------
Full Build: 179 sec 255689 lines
Inc Make  : 179 sec all files affected: 255689 lines

Notice that a full build is 16 seconds faster when I pre-compile my own header files, but look what happens when I do an incremental make after changing a header file. The incremental make takes just as long as a full build. When you pre-compile your own header files, the compiler rebuilds the pre-compiled image every time you change a header file. Additionally, when the pre-compiled image changes, every file that depends on that image will be re-compiled as well. So if you alter a header file, the entire project essentially gets rebuilt. When I do not pre-compile my own header files, the pre-compiled image never gets rebuilt. This keeps the incremental make time down, 28 seconds compared to 179 seconds.

Since you probably perform an incremental make 10 times more often than you do a full build, it seems wise to keep the incremental make time down, even if it means that a full build will be 10% slower. This is the approach that I take. I do not define the PRECOMPILE_ALL value in any of my projects.

Don't remove existing #include statements: Creating a common header file does not mean that you should remove include statements from your header files and C++ source files. Leave those include statements where they are. There are several reasons why you should leave existing include statements. First, if you remove include statements from your header files, C++Builder will simply add them back again. Second, you may want to stop pre-compiling certain files, which would force to you add the include statements back into your source files. Lastly, by leaving include statements intact, you preserve the necessary inclusion order between header files. If you remove include statements, you will need to worry about the order that you list include statements in your common header.

The include files prevent against multiple inclusion, so you don't need to worry about including the same file twice. The code below is taken from the mainform of the project from the Results section. It shows the include statements that remain in the source, even though the common header already includes them.

//----------------------------------------------------------------------
// MAINFORM.CPP
#include
#include "pch.h"
#pragma hdrstop

#include
#include "mainform.h"
#include "About.h"
#include "util.h"
#include "claim.h"
#include "expert.h"
#include "vendor.h"
#include "lawfirm.h"
#include "registry.h"
#include "exceptions.h"
...
...
//----------------------------------------------------------------------
// MAINFORM.H
#ifndef mainformH
#define mainformH
//----------------------------------------------------------------------
#include
#include
#include
#include
#include
#include
#include
#include
#include

#include "defs.h"

//----------------------------------------------------------------------
class TMDIParent : public TForm
{
__published:
...


The point of showing this code is to demonstrate that even though I use a common include file called PCH.H, I don't remove existing include statements from my source files. When I create a new source file, I add include statements so that file compiles without relying on includes from the common header. Once the new source file compiles, I insert an include statement for the common header to keep compile times down.

Observe case sensitivity for include statements: Someone posted a comment on the C++Builder IDE group that the compiler observes case sensitity when matching pre-compiled images. If you include the common header with varying case in different units, the compiler will regenerate the pre-compiled image for each one. The next code example demonstrates what you should not do.

//----------------------------------------------------------------------
// MAINFORM.CPP
#include
#include "pch.h"
#pragma hdrstop
...

//----------------------------------------------------------------------
// ABOUT.CPP
#include     // OK. same case as mainform.cpp
#include "pch.h"
#pragma hdrstop
...

//----------------------------------------------------------------------
// SPLASH.CPP
#include
#include "PCH.H"    // WRONG. mismatched case
#pragma hdrstop
...

//----------------------------------------------------------------------
// LOGON.CPP
#include     // WRONG. mismatched case
#include "pch.h"
#pragma hdrstop
...

In this example, the compiler will generate and use the same pre-compiled image for MAINFORM.CPP and ABOUT.CPP, but SPLASH.CPP and LOGON.CPP will each generate their own pre-compiled image, which will slow down the compile time. The rule of thumb is this: every include file listed above the #pragma hdrstop directive should use the same case that other files use. Include statements below the #pragma hdrstop directive don't have to match case, because they are not pre-compiled.

Consider adding VCL.H to the common header: The common header that was used in the example code for technique 2 does not include the file VCL.H. Each CPP source file includes both VCL.H and PCH.H, like this.

//-----------------------------------------------
#include
#include "pch.h"
#pragma hdrstop

You may prefer to include VCL.H from within the common header. If you do, then each CPP file can simply include the common header.

//-----------------------------------------------
#include "pch.h"
#pragma hdrstop

This is cleaner, and less prone to error because you don't have to worry about which file should be listed first. However, it violates the suggestion from the note don't remove existing #include statements. If you ever need to yank out the common header file, you will need to add VCL.H back into every CPP file in your project.

Use a separate CSM file for each project: By default, C++Builder creates a common pre-compiled header file called vcl50.csm in the $(BCB)lib directory (the name of this file is different with each version of C++Builder). C++Builder will share this pre-compiled header file among all of your projects. In order to take advantage of the techniques in this article, you should configure your projects to create and use their own pre-compiled image file.

You can do tell your project to use its own pre-compiled header file by specifying a file name for the pre-compiled image. This option is on the Compiler tab of the Project-Options dialog box. Change this value from $(BCB)libvcl50.csm to a filename that won't conflict with other projects, such as pch.csm.

There are several advantages to using a separate CSM file for each project. First, it allows you to create a customize your common header file for each project. Secondly, because the CSM file in the lib directory is shared, it tends to grow in size. It is not uncommon to have a shared CSM file that is larger than 30 MB. Lastly, some users have reported that sharing the same CSM across multiple projects is a source of phantom compiler errors. If you are getting strange compiler errors in algorithm.h, you might be able to solve the problem by not sharing your CSM files across multiple projects.

Count the number of #00 files in your project directory: When C++Builder generates a pre-compiled header image, it saves that image to a file with a .CSM extension. It will also save one or more files with an extension of .#?? (ie #00, #01, #02). The number of .#?? files depends on how well you have optimized your pre-compiled headers. If you optimize them perfectly, C++Builder will generate a single file with an extension of .#00. If you don't optimize your files correctly, you will see other files with similar file extenstions (.#01, .#02, etc). The presence of additional #00 files indicates that you have not optmized your headers correctly.

So what are these files? For each unique pre-compiled image in the .CSM file, C++Builder generates one file with a .#?? extension, starting with .#00. These files are usually between 1 and 2 MB's in size. When you have optimized your files perfectly, the .CSM file will contain one and only one pre-compiled image. As a result, you end up with one .CSM file and one file with a .#00 extension. When you don't optimize your projects, the .CSM file may contain many unique pre-compiled images. Each unique image generates one additional .#?? file.

Here are a couple of additional tips regarding these mysterious .#00 files. C++Builder generates these files as its needs them, but it never deletes them. Because these files are fairly large, you should delete them every so often. Also, because C++Builder never deletes them, you may see .#00 files even after you optimize your pre-compiled headers. Third, these files are created in the same directory that the CSM file resides. By default, this is the $(BCB)lib directory. Lastly, you may want to take a look in your $(BCB)lib and count how many of these files are lying around. In the previous note, we talked about how C++Builder generates one common .CSM file in the lib directory that all projects use. This shared .CSM file tends to be big, and it also tends to contain many different pre-compiled images. Since each unique image generates a separate .#00 file, you end up with tons of .#00 files in your lib directory. Currently, my BCB4 lib directory contains five of these files ( vcl40.#00,vcl40.#01, vcl40.#02, vcl40.#03, and vcl40.#04).

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/10752043/viewspace-987731/,如需转载,请注明出处,否则将追究法律责任。

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