Template function gives “no matching function for call” error

问题

First question on stackoverflow :) I'm relatively new to C++, and have never used templates, so forgive me if I'm doing something silly. I have a template function that combs through a list and checks for a specified element of a general type. That way I can specify whether it's looking for a string, or an int, or whatever.

template <class T>   
bool inList(T match, std::string list)  
{
    int listlen = sizeof(list);
    for (int i = 0; i <= listlen; i++) {
        if (list[i] == match) return true;
        else continue;
    }
    return false;
};

This is my call to inList(). testvec is a string vector with a few elements, including "test":

if (inList<string>("test", testvec))
    cout << "success!";
else cout << "fail :(";

To my dismay and confusion, upon compiling, I am slapped with the following error:

error: no matching function for call to 'inList(const char [5], std::vector<std::basic_string<char> >&)'

What am I doing incorrectly? :(

[EDIT] I neglected to mention that the template definition is in the global namespace. (it's a simple test program to see if my template will work, and it does not, apparently :( )

回答1:

That's because there is no way to convert from a std::vector to a std::string. Instead, what you need to do is abstract over the concept of a collection.

You do this so that people can pass in any collection type they want. They can use an array, a vector, a string, a list, a deque... as long as it fits the 'concept' of a collection (here's to hoping c++1x comes with concepts!). They can even use their own in-house specially optimized collection types. That's the beauty of templates.

Using C++11 (works with any standard collection, primitive arrays, and user-defined types):

template<class elem_t, class list_t>
bool in_list(const elem_t& elem, const list_t& list) {
   for (const auto& i : list) {
      if (elem == i) {
         return true;
      }
   }
   return false;
}

EDIT: It's a non-standard extension to deduce std::initializer_list as a template argument, so provide an explicit override:

template<class elem_t>
bool in_list(const elem_t& elem, std::initializer_list<elem_t> list) {
   for (const auto& i : list) {
      if (elem == i) {
         return true;
      }
   }
   return false;
}

With this version, you can call it like:

int main() {
   std::vector<int> a = {1, 2, 3, 4, 5};
   std::cout << in_list(3, a) << std::endl;
   std::string b = "asdfg";
   std::cout << in_list('d', b) << std::endl;
   std::cout << in_list('d', "asdfg") << std::endl;
   std::cout << in_list(3, {1, 2, 3, 4, 5}) << std::endl;
   return 0;
}

And for those of us still in C++98, this will work for both strings and vectors, and some user-defined types. It will not work with raw arrays though.

template<class elem_t, class list_t>
bool in_list_98(const elem_t& elem, const list_t& list) {
   list_t::const_iterator end = list.end(); //prevent recomputation of end each iteration
   for (list_t::const_iterator i = list.begin(); i < end; ++i) {
      if (elem == *i) {
         return true;
      }
   }
   return false;
}

Or, you can go STL style:

template<class elem_t, class iterator_t>
bool in_list_stl(const elem_t& elem, iterator_t begin, iterator_t end) {
   for (iterator_t i = begin; i < end; ++i) {
      if (elem == *i) {
         return true;
      }
   }
   return false;
}
//call like std::string s = "asdf"; in_list_stl('s', s.begin(), s.end());

If I made a mistake, sorry, I don't have my compiler running right now...

来源：https://stackoverflow.com/questions/10369464/template-function-gives-no-matching-function-for-call-error

标签

c++

templates

compiler-errors

function-calls

function-call