模板的选择性编译

// file.h
template<typename T> class object { /* lots of code */ };

// file.inc
template<typename T>
object::object(const T*data) { /* ... */ }
    // and more: definitions of all non-inline functionality of class object<>

// file.cc to be compiled and linked
#include "file.h"
#include "file.inc" 

template struct<int>;
template struct<double>;

// user.cc
#include "user.h"
#include "file.h"

object<double> x{0.4}                              // okay: uses pre-compiled code
object<user_defined> z(user_defined{"file.dat"});  // error: fails at linking

如果用户 #include s码 "file.inc"

// user.cc
#include "user.h"
#include "file.h"
#include "file.inc"

object<double>  x{0.4}                            // error: duplicate code 
object<user_defined> z(user_defined{"file.dat"}); // okay

file.cc .