Server body 21: pre compilation processing by different compilers (a brief introduction to MSVC and GCC)

One 、 background

• Most compilers support precompiling
  In most of c/c++ Compilers support precompiled headers , for example ：gcc,clang,msvc etc.
• Precompile function
  Used to optimize c++ Speed of code compilation , After all c++ If the header file contains the template definition , Compilation speed is very slow
• Precompiled approach
  Most general header files can be placed in one header.h in , Compile other source code before compiling it , Later code can reuse this part of the precompiled header , It can greatly reduce the frequent redundant compilation of header files .

Two 、msvc Precompiled header processing for

Precompiled header in msvc Is very common in projects , I often see something like stdafx.cpp, stdafx.h The file of , For this purpose , and msvc The compiler compiles stdafx.cpp To generate precompiled header files stdafx.pch Of .

1） summary

Generally, under the same project stdafx.cpp Is to create a precompiled header （/Yc）, other .cpp Files are using precompiled headers （/Yu）, all .h The file doesn't care about this

2） Specific use

• Overall process Preview
  1) Create precompiled header command
  2) Other files use this stdafx.pch Methods
  3) link obj Attention
  4) Differences with other compilers

• Create precompiled header command

$ cl.exe -c -Yc -Fpstdafx.pch -Fostdafx.obj stdafx.cpp

1）-Yc Is to create a precompiled header stdafx.pch
2）-Fp To specify the *.pch Output file path of 
3）-Fo Specify compile stdafx.cpp Generate object file 

• How to use this for other files stdafx.pch？

By way of stdafx.h Pass in -Yu To tell the compiler , Compile the current code , Ignore #include “stdafx.h”, Directly use the compiled stdafx.pch file .

cl.exe -c -Yustdafx.h -Fpstdafx.pch -Fotest.obj test.cpp

• At the end of the link
  Need to put ：stdafx.obj, test.obj All connected , This is also related to gcc, clang The compiler is different .

link.exe -out:test test.obj stdafx.obj

• Other considerations
  1） notes ： Be sure stdafx.obj Also on the link , although stdafx.cpp Only for generating stdafx.pch, But object files are also required .
  2） There's another one with gcc, clang The difference is ,msvc Of -Yu Appoint stdafx.h Must be #include "stdafx.h" Header file name in , Not a file path .

3、 ... and 、gcc Precompiled header file processing for

1）gcc Search for stdafx.pch Rules for file paths

1） from stdafx.h In the directory , lookup stdafx.h.pch Does the file exist 
2） from -I The header file search path of find find find stdafx.h.pch

2） compile

• Overall process

1) Compile header file generation pch file ：
2) Using precompiled header files ：

• Compile header file generation pch file ：

gcc -c -o stdafx.pch stdafx.h

• Using precompiled header files ：

gcc -c -include stdafx.h -o test.o test.cpp

Four 、 remarks （C and C++ about *.h The difference between compilation ）

1）gcc、clang about *.h Header file compilation , The default is as c Used by precompiled headers , This one c++ Of pch It's different .
2）*.h The header file compilation of cannot be given c++ The code uses , If you want to generate c++ Usable pch file , You have to tell the compiler , How to compile stdafx.h

• Method
  This can be done through -x c+±header Parameters to solve ：

gcc -c -x c++-header -o stdafx.pch stdafx.h

• Of course, it can also be solved by modifying the suffix ：

gcc -c -o stdafx.pch stdafx.hpp