C ++ 17中的歧义错误(模板模板参数和默认参数问题) [英] Ambiguity error in C++17 (template template parameters and default arguments issue)

问题描述

我有一些代码，这些代码由g ++用c++14和c++17标准标志来解释:

#include <iostream>
#include <vector>

template<class T, class A>
void func(const std::vector<T, A>&v)
{
    std::cout << 1 << std::endl;
}

template<typename T, template <typename>class Vector>
void func(const Vector<T>&v)
{
    std::cout << 2 << std::endl;
}

void f()
{
    std::vector<int> v;
    func(v);
}

int main()
{
    f();
    return 0;
}

当我尝试使用命令编译此代码

g ++ -std = c ++ 14 -Wall -pedantic main.cpp

一切正常.

但是当我尝试使用命令编译此代码时

g ++ -std = c ++ 17 -Wall -pedantic main.cpp

我收到此错误:

main.cpp: In function 'void f()':
main.cpp:19:11: error: call of overloaded 'func(std::vector<int>&)' is ambiguous
     func(v);
           ^
main.cpp:5:6: note: candidate: 'void func(const std::vector<_Tp, _Alloc>&) [with T = int; A = std::allocator<int>]'
 void func(const std::vector<T, A>&v)
      ^~~~
main.cpp:11:6: note: candidate: 'void func(const Vector<T>&) [with T = int; Vector = std::vector]'
 void func(const Vector<T>&v)

从C ++ 17标准的角度来看，我无法弄清楚这段代码有什么问题.

解决方案

自C ++ 17以来，行为已更改.

在C ++ 17之前，该代码有效，因为 std::vector 有两个模板参数(第二个具有默认参数std::allocator<T>)，而模板模板参数Vector被声明为只有一个，它们不匹配，因此将不考虑第二个func. /p>

从C ++ 17开始( CWG 150 )，则允许模板模板参数使用默认模板参数将模板模板参数与更少的模板参数进行匹配.这意味着func都将成为有效候选人，然后导致模棱两可.

template<class T> class A { /* ... */ };
template<class T, class U = T> class B { /* ... */ };

template<template<class> class P> class X { /* ... */ };

X<A> xa; // OK
X<B> xb; // OK in C++17 after CWG 150
         // Error earlier: not an exact match

I have code which is differently interpreted by g++ with the c++14 and c++17 standard flags:

#include <iostream>
#include <vector>

template<class T, class A>
void func(const std::vector<T, A>&v)
{
    std::cout << 1 << std::endl;
}

template<typename T, template <typename>class Vector>
void func(const Vector<T>&v)
{
    std::cout << 2 << std::endl;
}

void f()
{
    std::vector<int> v;
    func(v);
}

int main()
{
    f();
    return 0;
}

When I'm trying compile this code with command

g++ -std=c++14 -Wall -pedantic main.cpp

everything works just fine.

But when I'm trying to compile this code with command

g++ -std=c++17 -Wall -pedantic main.cpp

I get this error:

main.cpp: In function 'void f()':
main.cpp:19:11: error: call of overloaded 'func(std::vector<int>&)' is ambiguous
     func(v);
           ^
main.cpp:5:6: note: candidate: 'void func(const std::vector<_Tp, _Alloc>&) [with T = int; A = std::allocator<int>]'
 void func(const std::vector<T, A>&v)
      ^~~~
main.cpp:11:6: note: candidate: 'void func(const Vector<T>&) [with T = int; Vector = std::vector]'
 void func(const Vector<T>&v)

I can't figure out what is wrong with this code from the C++17 standard's point of view.

解决方案

The behavior changed since C++17.

Before C++17, the code works because std::vector has two template parameters (the 2nd one has the default argument std::allocator<T>), while the template template parameter Vector is declared to have only one, they don't match then the 2nd func won't be considered.

Since C++17 (CWG 150), the default template arguments are allowed for a template template argument to match a template template parameter with fewer template parameters. That means both func become valid candidates and then leads to ambiguity.

template<class T> class A { /* ... */ };
template<class T, class U = T> class B { /* ... */ };

template<template<class> class P> class X { /* ... */ };

X<A> xa; // OK
X<B> xb; // OK in C++17 after CWG 150
         // Error earlier: not an exact match

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