ostream& 运算符的显式模板参数规范<< 令人费解的编译错误

Question

我有以下代码：

#include <iostream>
//#include <algorithm> // compile error when `g++ -std=c++11`, fine otherwise

using namespace std;

template<typename T>
class Foo
{
    T x_;
public:
    Foo(const T& x={}): x_(x){}; // default ctor, bounded rvalue default-initialized

    template<typename S>
    friend ostream& operator<<(ostream& lhs, const Foo<S>& rhs)
    {
        return lhs << rhs.x_;
    }
};

int main()
{
    Foo<double> foo(10.1);
    cout << foo << endl;
    operator<<(cout, foo) << endl;

    //puzzling compile-time error when #include<algorithm> and compile 
    // with `g++ -std=c++11`, otherwise fine (both clang++ and g++)
    operator<< <double>(cout, foo) << endl; 
}

一切都可以编译并且工作正常，但是，如果我 #include<algorithm> 并使用 g++ -std=c++11 编译，我会收到一个非常混乱的编译错误（见下文，与 std::uniform_int_distribution 相关？！？！）。它发生在最后一行，当我为调用operator<< 明确指定类型double 时。

不管-std=c++11 标志如何，它都可以在clang++ 上工作，也可以在没有-std=c++11 标志的g++ 上工作。这里发生了什么？我真的不知道，我是否以某种方式重载了algorithm 中定义的全局<<？ 这是g++ 错误吗？ （我使用 g++4.9 顺便说一句）。

In file included from /opt/local/include/gcc49/c++/random:49:0,
                 from /opt/local/include/gcc49/c++/bits/stl_algo.h:66,
                 from /opt/local/include/gcc49/c++/algorithm:62,
                 from /Users/vlad/minimal.cpp:2:
/opt/local/include/gcc49/c++/bits/random.h: In instantiation of 'class std::uniform_int_distribution<double>':
/Users/vlad/minimal.cpp:25:34:   recursively required by substitution of 'template<class _IntType, class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::uniform_int_distribution<_IntType>&) [with _IntType = double; _CharT = char; _Traits = std::char_traits<char>]'
/Users/vlad/minimal.cpp:25:34:   required from here
/opt/local/include/gcc49/c++/bits/random.h:1668:7: error: static assertion failed: template argument not an integral type
       static_assert(std::is_integral<_IntType>::value,
       ^
/opt/local/include/gcc49/c++/bits/random.h: In instantiation of 'class std::geometric_distribution<double>':
/Users/vlad/minimal.cpp:25:34:   recursively required by substitution of 'template<class _IntType, class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::geometric_distribution<_IntType>&) [with _IntType = double; _CharT = char; _Traits = std::char_traits<char>]'
/Users/vlad/minimal.cpp:25:34:   required from here
/opt/local/include/gcc49/c++/bits/random.h:4010:7: error: static assertion failed: template argument not an integral type
       static_assert(std::is_integral<_IntType>::value,
       ^
/opt/local/include/gcc49/c++/bits/random.h: In instantiation of 'class std::negative_binomial_distribution<double>':
/Users/vlad/minimal.cpp:25:34:   recursively required by substitution of 'template<class _IntType, class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::negative_binomial_distribution<_IntType>&) [with _IntType = double; _CharT = char; _Traits = std::char_traits<char>]'
/Users/vlad/minimal.cpp:25:34:   required from here
/opt/local/include/gcc49/c++/bits/random.h:4210:7: error: static assertion failed: template argument not an integral type
       static_assert(std::is_integral<_IntType>::value,
       ^
/opt/local/include/gcc49/c++/bits/random.h: In instantiation of 'class std::poisson_distribution<double>':
/Users/vlad/minimal.cpp:25:34:   recursively required by substitution of 'template<class _IntType, class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::poisson_distribution<_IntType>&) [with _IntType = double; _CharT = char; _Traits = std::char_traits<char>]'
/Users/vlad/minimal.cpp:25:34:   required from here
/opt/local/include/gcc49/c++/bits/random.h:4432:7: error: static assertion failed: template argument not an integral type
       static_assert(std::is_integral<_IntType>::value,
       ^
/opt/local/include/gcc49/c++/bits/random.h: In instantiation of 'class std::binomial_distribution<double>':
/Users/vlad/minimal.cpp:25:34:   recursively required by substitution of 'template<class _IntType, class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::binomial_distribution<_IntType>&) [with _IntType = double; _CharT = char; _Traits = std::char_traits<char>]'
/Users/vlad/minimal.cpp:25:34:   required from here
/opt/local/include/gcc49/c++/bits/random.h:3779:7: error: static assertion failed: template argument not an integral type
       static_assert(std::is_integral<_IntType>::value,
       ^
/opt/local/include/gcc49/c++/bits/random.h: In instantiation of 'class std::discrete_distribution<double>':
/Users/vlad/minimal.cpp:25:34:   recursively required by substitution of 'template<class _IntType, class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::discrete_distribution<_IntType>&) [with _IntType = double; _CharT = char; _Traits = std::char_traits<char>]'
/Users/vlad/minimal.cpp:25:34:   required from here
/opt/local/include/gcc49/c++/bits/random.h:5253:7: error: static assertion failed: template argument not an integral type
       static_assert(std::is_integral<_IntType>::value,
       ^

Answer 1

我认为归结为：

#include <type_traits>

template<class T>
struct foo
{
    static_assert(std::is_integral<T>{}, "T must be integral");
};

template<class T> struct bar {};

template<class T, class Stream>
Stream& operator<<(Stream& p, foo<T> const&);

template<class T, class Stream>
Stream& operator<<(Stream& p, bar<T> const&);

int main()
{
    int x;
    operator<< <double>(x, 1.0);
}

请注意，在 libstdc++ 的实现中，显式传递给 operator<< 的第一个模板参数用作分发的模板参数（在 OP 中）。所以理论上，从operator<< 调用的第二个函数参数到分布类型的隐式转换是可能的。

在检查歧义之前，重载解析必须实例化类模板以检查转换构造函数：如果有，它们是否是显式的等等——也就是说，如果转换是可行的 em>。

然而，为非整数类型实例化类模板是一个错误，由static_assert 检查。因此，在选择其中一个重载之前，编译器会抱怨实例化。

如果编译器不需要它来选择重载，则不严格要求此实例化，请参阅 [temp.inst]/7。所有其他重载需要用户定义的转换（参数类型是类，不同于参数类型），而在 OP 中定义为友元函数的重载是精确匹配。因此，不需要检查其他转换的可行性，无论如何都不会选择这些重载。

不实例化不需要的类模板是某种优化——它不是强制性的。 clang++ 似乎在这里执行它，g++ 没有（并实例化类模板）。另一个例子和一些讨论，见C++ inconsistency between gcc and clang