深入理解模板

模板有两类：函数模板和类模板。

无类型模板参数

默认模板参数

模板类型的模板参数

函数模板重载

编译时断言

//: C05:StaticAssert1.cpp {-xo}

// A simple, compile-time assertion facility

#define STATIC_ASSERT(x) \

  do { typedef int a[(x) ? 1 : -1]; } while(0)

int main() {

  STATIC_ASSERT(sizeof(int) <= sizeof(long)); // Passes

  STATIC_ASSERT(sizeof(double) <= sizeof(int)); // Fails

} ///:~

<style type="text/css">.csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } </style>

前面的内容说明了如何对编译时布尔表达式求值，。在效仿编译时断方面剩下的问题就是打印一个有意义的错误消息并且停止编译。所有的编译错误都要求编译器停止编译。解决这个问题的一个技巧是在错误消息中插入有用的文本。

//: C05:StaticAssert2.cpp {-g++}

#include <iostream>

using namespace std;

// A template and a specialization

template<bool> struct StaticCheck {

  StaticCheck(...);

};

template<> struct StaticCheck<false> {};

// The macro (generates a local class)

#define STATIC_CHECK(expr, msg) {             \

  class Error_##msg {};                       \

  sizeof((StaticCheck<expr>(Error_##msg()))); \

}

// Detects narrowing conversions

template<class To, class From> To safe_cast(From from) {

  STATIC_CHECK(sizeof(From) <= sizeof(To),

               NarrowingConversion);

  return reinterpret_cast<To>(from);

}

int main() {

  void* p = 0;

  int i = safe_cast<int>(p);

  cout << "int cast okay” << endl;

  //! char c = safe_cast<char>(p);

} ///:~

<style type="text/css">.csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } </style>

以上的代码理解起来比较困难，一个简单的方法是先展开

int i = safe_cast<int>(p);相当于

{                                                   

class Error_NarrowingConversion {};            

sizeof( StaticCheck<sizeof(void*) <= sizeof(int)>(Error_NarrowingConversion() ) );      

}

在这儿，sizeof(void*) <= sizeof(int)为模板类StaticCheck的参数，若其值为TRUE，则调用通用构造函数，否则，调用特化的类，此特化的类为一个空类，

无法接受参数Error_NarrowingConversion()，因此编译将出错。

<style type="text/css">.csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } </style>

表达式模板：