Why use precompiled headers (C/C++)?

Question

Why use precompiled headers?

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Reading the responses, I suspect what I've been doing with them is kind of stupid:

#pragma once  // Defines used for production versions  #ifndef PRODUCTION #define eMsg(x) (x) // Show error messages #define eAsciiMsg(x) (x) #else #define eMsg(x) (L"") // Don't show error messages #define eAsciiMsg(x) ("") #endif // PRODUCTION  #include "targetver.h" #include "version.h"  // Enable "unsafe", but much faster string functions #define _CRT_SECURE_NO_WARNINGS #define _SCL_SECURE_NO_WARNINGS  // Standard includes #include <stdio.h> #include <tchar.h> #include <iostream> #include <direct.h> #include <cstring> #ifdef _DEBUG #include <cstdlib> #endif  // Standard Template Library #include <bitset> #include <vector> #include <list> #include <algorithm> #include <iterator> #include <string> #include <numeric>  // Boost libraries #include <boost/algorithm/string.hpp> #include <boost/lexical_cast.hpp> #include <boost/scoped_array.hpp>  //Windows includes #define WIN32_LEAN_AND_MEAN #include <windows.h> #include "FILETIME_Comparisons.h" #include <shlwapi.h> #include <Shellapi.h> #include <psapi.h> #include <imagehlp.h> #include <mscat.h> #include <Softpub.h> #include <sfc.h> #pragma comment(lib, "wintrust.lib") #pragma comment(lib,"kernel32.lib") #pragma comment(lib,"Psapi.lib") #pragma comment(lib,"shlwapi.lib") #pragma comment(lib,"imagehlp.lib") #pragma comment(lib,"Advapi32.lib") #pragma comment(lib,"Shell32.lib") #pragma comment(lib,"Sfc.lib") #pragma comment(lib,"Version.lib")  // Crypto ++ libraries #ifdef _DEBUG #pragma comment(lib,"cryptlibd.lib") #else #pragma comment(lib,"cryptlib.lib") #endif #define CRYPTOPP_ENABLE_NAMESPACE_WEAK 1 #include <md5.h> #include <sha.h>  // String libraries #include "stringUnicodeConversions.h" #include "expandEnvStrings.h" #include "randomString.h" #include "getShortPathName.h"  // Regular Expression Libraries #include "fpattern.h"  // File Result Record #include "unixTimeToFileTime.h" #include "fileData.h"  // Writer #include "writeFileData.h"  // Criteria Structure System #include "priorities.h" #include "criterion.H" #include "OPSTRUCT.H" #include "regexClass.H" #include "FILTER.h"  // Sub Programs Root Class #include "subProgramClass.h"  // Global data #include "globalOptions.h"  // Logger #include "logger.h"  // Console parser #include "consoleParser.h"  // Timeout handler #include "timeoutThread.h"  // Zip library #include "zip.h" #include "unzip.h" #include "zipIt.h"  // Scanner #include "mainScanner.h" #include "filesScanner.h"  // Sub Programs #include "volumeEnumerate.h" #include "clsidCompressor.h" #include "times.h" #include "exec.h" #include "uZip.h"  // 64 bit support #include "disable64.h" 

Answer 1

In C/C++, the #include mechanism is a textual copy of the file specified into the current file. Headers include other headers (which include yet other headers), so when you do a #include, it could be adding tens of thousands of lines of C++ into each cpp file (or cxx, c, whatever), all of which need to be compiled each time. This can be a severe bottleneck for large projects.

Precompiled headers speed this up by compiling each header once, then including that compiled state into the cpp they are included in.

Answer 2

It compiles a lot quicker. C++ compilation takes years without them. Try comparing some time in a large project!