使用模板化构造函数解释为函数定义的非模板化类的实例? [英] Instance of a non-templated class using templated constructor interpreted as a function definition?

问题描述

以下代码最能说明我的问题:

My issue is best demonstrated by the following code:

#include <fstream>
#include <iterator>

class Bar
{
    public: template <class Iterator> Bar(Iterator first, Iterator last) {}
};

void foo(const Bar& bar) { }

int main(int argc, char** argv)
{
    std::ifstream file("file.txt");

    Bar bar(std::istream_iterator<char>(file), std::istream_iterator<char>());

    foo(bar); // error C2664: 'foo' : cannot convert parameter 1 from 'Bar (__cdecl *)(std::istream_iterator<_Ty>,std::istream_iterator<_Ty> (__cdecl *)(void))' to 'const Bar &'
              // with
              // [
              //     _Ty=char
              // ]
              // Reason: cannot convert from 'overloaded-function' to 'const Bar'
              // No constructor could take the source type, or constructor overload resolution was ambiguous

    return 0;
};

以下是一些类似的 bar 实例，不会导致同样的歧义:

Here are some similar instantiations of bar that don't cause the same ambiguity:

Bar bar = Bar(std::istream_iterator<char>(file), std::istream_iterator<char>());

和

std::istream_iterator<char> start(file);
std::istream_iterator<char> end;
Bar bar(start, end);

我的问题是，导致它被误解的 bar 的第一个声明是什么?

My question is, what is it about the first declaration of bar that causes it to be misinterpreted?

注意:我正在使用 Visual Studio 2010 (10.0.30319.1) 进行测试

note: I'm testing with Visual Studio 2010 (10.0.30319.1)

推荐答案

ArunMu 获得部分功劳，它确实是 最令人烦恼的解析，但该术语是在 Meyer 的 Effective STL(第 1 章，第 6 项)中创造的，而不是 Exceptional C++.

ArunMu gets partial credit, it is indeed an example of Most Vexing Parse, but that term was coined in Meyer's Effective STL (Chapter 1, Item 6) not Exceptional C++.

它是被解释为一个函数指针(错误的(__cdecl *)部分是一个致命的放弃)，显然C++标准要求它被这样解释.有人对此有章节/诗句引文吗?

It is being interpreted as a function pointer (the (__cdecl *) portion of the error is a dead give away), and apparently the C++ standard requires it to be interpreted that way. Does anyone have a chapter/verse citation for that?

还有另一种解决方案来消除歧义.在每个参数周围添加一组额外的括号也有效:

There is also a another solution to provide a disambiguation. Adding an additional set of parenthesis around each parameter works too:

Bar bar( (std::istream_iterator<char>(file)), (std::istream_iterator<char>()) );

同样值得指出的是，这个问题与我最初认为的模板无关.

It's also worth pointing out that the issue is unrelated to the templates, as I had originally thought.

这篇关于使用模板化构造函数解释为函数定义的非模板化类的实例?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！