我想知道为什么下面的代码在 gcc 中运行得很好

#include <iostream>
using namespace std;

template<typename T>
struct F { 
  static T const value;
};

template<> 
struct F<int> { // Specialization
    static int const value; 
}; 

template struct F<int>;

template<typename T>
T const F<T>::value = sizeof(T);

template<>
int const F<int>::value = 42;

int main() {

    struct F<int> ma;
    cout << ma.value;

    return 0;
}

http://ideone.com/wvrurz

在 MSVC 2012 上我无法编译它:

#include <iostream>
using namespace std;

template<typename T>
struct F {
  static T const value;
};

template<> 
struct F<int> { // Specialization
    static int const value; 
};

//template struct F<int>; // error C2950: 'F<int>' : cannot explicitly instantiate an explicit specialization

template<typename T>
T const F<T>::value = sizeof(T);

//template<>
//int const F<int>::value = 42; // error C2998: 'const int F<int>::value' : cannot be a template definition

int main() {

    struct F<int> ma;
    cout << ma.value;

    return 0;
}

根据我在 n3242 §14.7 5 中阅读的内容

both an explicit instantiation and a declaration of an explicit specialization shall not appear in a program unless the explicit instantiation follows a declaration of the explicit specialization.

我相信是这样的。我错过了什么吗？

最佳答案

太长；没读过

• msvc 2012 正确拒绝标记为 // error C2998 的行,

• 然而，之前的诊断是错误的，应该被接受；因为它在 newer versions编译器。

^{注意:可以找到与C2950相关的错误报告，here} .

关于C2950

msvc 2012 对有问题的行发出诊断是错误的。

template<class T> struct A;

template<>
struct A<int> { };

template struct A<int>; // legal

int main () { }

标准规定显式实例化应该包含一个simple-template-id，这正是A<int>是，并且是这样说的；它是合法的 C++。

14.6.2p3 Explicit instantiation [temp.explicit]

If the explicit instantiation is for a class or member class, the elaborated-type-specifier in the declaration shall include a simple-template-id.

14.2p1 Names of template specializations [temp.names]

A template specialization (14.7) can be referred to by a template-id:

simple-template-id:
         template-name < template-argument-list_opt >

措辞变化:C++03 vs C++11

14.7.2p5 有一些从 C++11 开始的新措辞，这些措辞是在以下缺陷报告之后实现的:

• 259. Restrictions on explicit specialization and instantiation

14.7.2p5 Explicit instantiation [temp.explicit]

For a given set of template arguments, if an explicit instantiation of a template appears after a declaration of an explicit specialization for that template, the explicit instantiation has no effect.

^{Note: Kudos to @dyp for bringing attention to the previously linked DR.}

关于C2998

这个错误是准确的；你指的不是依赖于模板参数的东西，这意味着你不应该使用 template<>关于所讨论的定义。

较新版本的 gcc发出有关它的诊断，clang 正确地拒绝了这样的定义。

template<class T> struct A;

template<>
struct A<int> { 
  static int const value;
};

template<> int const A<int>::value = 42; // ill-formed, `value` does not depend on
                                         //              any template parameter since it's
                                         //              explicitly a part of `A<int>`

int main () { }

gcc   => foo.cpp:8:22: warning: too many template headers for A<int>::value (should be 0)
clang => foo.cpp:8:1: error: extraneous 'template<>' in declaration of variable 'value'
msvc  => foo.cpp(8) : error C2998: 'const int A<int>::value' : cannot be a template definition

以上诊断正确。

有问题的行违反了标准的以下部分:

14.7.3p5 Explicit specialization [temp.expl.spec]

Members of an explicitly specialized class template are defined in the same manner as members of normal class, and not using the template<>syntax. The same is true when defining a member of an explicitly specialized member class.

关于c++ - MSVC 中的模板静态定义和显式特化实例化错误，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24333686/