C++ Compiler Macro for Status of “Use Precompiled

问题:

It there a predefined c++ compiler macro that I can use to tell, whether a file is compiled with "Use Precompiled Headers", "Create Precompiled Headers", "Dont Use Precompiled Headers"?

See @IronMensan 's answer for the purpose of such a macro!

回答1:

I don't think there is anything, though I certainly understand the desire for one. Whenever I have to build my cross-platform library on a system that dozen't support PCH, it takes forever since a lot of files are pulling in way more than they really need and it would be nice to trim that out. Unfortunately I can't because of how Visual Studio handles PCH. Namely that the inclusion of the PCH must be the first non-comment line of the file. From the way you worded your question, I suspect that you are also working with Visual Studio.

I am not sure if this will work for you but you could try something like this:

#include MY_PCH_FILE

And use /DMY_PCH_FILE="myfile.h"

on the command line to control what the first include file is. After that you have full control over what gets included and proper header guards along with the optimization in most modern compilers to detect header guards could reduce build times. You can change the definition of the macro for individual file in the build settings of your project, in a similar manor to how you can change the PCH settings for each file.

Though I must admit that I am not sure what you are trying to do and I suspect this is really an XY problem

回答2:

Visual Studio/MSC does not provide a predefined macro that carries the setting of the /Y[-cdu] compiler switch for inspection from source code.

However, there is a solution to the problem you are trying to solve, i.e. controlling whether or not the first non-comment line of a source file should be #include "<my pch.h>": MSC offers the /FI (Name Forced Include File) compiler switch.

This option has the same effect as specifying the file with double quotation marks in an #include directive on the first line of every source file specified on the command line [...]

This compiler switch can either be specified on the compiler's command line, or on a per-project basis through the IDE's GUI (Project -> Properties: C/C++ -> Advanced: Forced Include File).

With a combination of the /Y[-cdu] and /FI compiler switches you can both control the use and meet the requirements for using precompiled headers, from outside the source code.

回答3:

In this case, I think you can create manualy yourself the macro. You can define USE_PRECOMPILEDHDR and FORCED_INCLUDEHDR when you use precompilation like this

#if USE_PRECOMPILEDHDR
    #ifndef FORCED_INCLUDEHDR
        #include "stdafx.h"
    #endif
#else 
    //..manualy include all your headers
#endif

But as other saying, except if you change for another compiler, you have no reason to use guards for this.

回答4:

This feature is unlikely to exist. The whole point of precompiled headers is that the headers will be compiled with exactly the same compiler options as when compiling for real. If the compiler were to offer a way for your code to tell the difference, then you could make your code behave differently (at a preprocessor level) depending on whether the compiler is precompiling or actually compiling.

回答5:

If you're looking to include header files based on whether or not precompiled headers are enabled, you should use an Include Guard instead.