Definition of C++ precompiled Headers -

please i've several questions : precompiled headers ? usage ? how make 1 ? , how include 1 ?

precompiled headers (pch short) compilers support. support , contain [aside "something can read quicker original header file"] each compiler producer decide. have little bit of understanding of how clang it's precompiled headers, , it's binary form of "parsed" c or c++ code in header - produces single file doesn't need parsing [to same level header file itself].

the purpose reduce compile-time. however, in experience, long part of compilation typically code-generation optimisation. however, in instances, when lots of header-files involved, time read , parse header files can noticeable part of overall compilation time.

generally speaking, how used tell compiler want precompiled header, , each compilation, compiler generate precompiled header if it's not there, , read in when present [1] - commonly done 1 named header file, includes lots of other things. microsoft visual studio typically has file called "stdafx.h" precompiled - , @ least in case of ms products, has first file inclduded in project [this no other header file example changes meaning of macro - expect there hash of compiler/command-line definitions of macros, if 1 of changes, pch recompiled].

the idea not include every single header-file in 1 precompiled file, header files used in files, , not changing (the pch needs regenerated if 1 if files precompiled has changed, it's no point in doing if keep changing header-files frequently). of course, other build dependency, using precompiled header need rebuilt if pch has changed.

for how use this, need read documentation compiler using.

[1] if nothing has changed requires rebuilt.