模板参数推导/替换失败 [英] template argument deduction/substitution failed

问题描述

我想实现一个容器，它可以通过为构造函数提供值序列来接受初始元素列表.

I want to implement a container which can accept initial list of elements by supplying the constructor with sequence of values.

  9 template <typename C, int max>
 10 struct c_array
 11 {
 12     typedef C value_type;
 13 
 14     typedef C* iterator;
 15     typedef const C* const_iterator;
 16 
 17     typedef C& reference;
 18     typedef const C& const_reference;
 19 
 20     c_array() { }
 21     template <class T>
 22         c_array(typename T::iterator begin,typename T::iterator end)
 23         {
 24         }
 25 
 26     C v[max];
 27     operator C*() { return v; }
 28 
 29     reference operator[] (ptrdiff_t i) { return v[i]; }
 30     const_reference operator[] (ptrdiff_t i) const { return v[i]; }
 31 
 32     iterator begin() { return v; }
 33     const_iterator begin() const { return v; }
 34 
 35     iterator end() { return v+max; }
 36     const_iterator end() const { return v+max; }
 37 
 38     size_t size() const { return max; }
 39 };
 40 
 41 int main(int argc, char** argv)
 42 { 
 43     std::vector<int> myvector(10,10);
 44     c_array<int,10> myarray1(myvector.begin(),myvector.end());
        ... 

编译时出现以下错误

 ...
 test.cc:56:61: error: no matching function for call to ‘c_array<int, 10>::c_array(std::vector<int>::iterator, std::vector<int>::iterator)’
 test.cc:56:61: note: candidates are:
 test.cc:22:9: note: template<class T> c_array::c_array(typename T::iterator, typename T::iterator)
 test.cc:22:9: note:   template argument deduction/substitution failed:
 test.cc:56:61: note:   couldn't deduce template parameter ‘T’
 ...

提前致谢，

推荐答案

在声明中

template <class T>
c_array(typename T::iterator begin,typename T::iterator end)

T 以一种称为非推导上下文"的方式使用.没有合理的方法，如果您传入 MyCustomIter 类型的参数，编译器会猜测什么类型 T 可能包含 typedef MyCustomIter iterator;.所以 C++ 标准说编译器甚至不应该尝试.

T is used in a way called a "non-deduced context". There's no reasonable way, if you pass in arguments of type MyCustomIter, for the compiler to guess what type T might contain a typedef MyCustomIter iterator;. So the C++ Standard says compilers shouldn't even try.

相反，您可以编写函数来接受任何类型并假设/记录它应该是一个迭代器.

Instead you can just write the function to take any type and assume/document that it should be an iterator.

template <class InputIter>
c_array(InputIter begin, InputIter end) {
    std::copy_n(begin, max, v);
}

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