C++ passing template type as argument gives error

Question

I have the following code, the problem is when I try to pass basic_string type to writeContainer function it gives me error it read the type Cont as std::_String_val< std::_Simple_types > so it give me errors like there is no size() method, and does not have end() or begin() methods for for each loop.

the nice thing, when I use vector it works fine, even though they are the same concept !! any help appreciated

template< template<typename> class Cont, typename T >
void writeContainer(Stream& stream, const Cont<T>& outValue) {
    stream << (int32_t)outValue.size(); 
    for (auto& v : outValue) {
        stream << v;
    }
}

template<typename T> 
Stream& operator<<(Stream& stream, const basic_string<T>& outValue) {
    writeContainer(stream, outValue); 
    return stream; 
}

errors I get, I use VS2013

error C2039: 'size' : is not a member of 'std::_String_val<std::_Simple_types<char>>'
see reference to function template instantiation 'void  writeContainer<std::_String_val,std::_Simple_types<char>>(Stream &,const std::_String_val<std::_Simple_types<char>> &)' being compiled
see reference to function template instantiation 'Stream &operator <<<char>(Stream &,const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &)' being compiled
error C3312: no callable 'begin' function found for type 'const std::_String_val<std::_Simple_types<char>>'
error C3312: no callable 'end' function found for type 'const std::_String_val<std::_Simple_types<char>>'
error C2065: 'v' : undeclared identifier

Answer 1

For a template template parameter, the argument must be a class template with the exact same number of parameters - counting parameters that have defaults. Thus, even though std::vector can be instantiated with one argument, it's a two-parameter template (the second parameter has a default), and cannot be an argument for Cont . Similarly, std::basic_string is a three-parameter template.

What happens in your example is this. In this particular implementation, std::basic_string is derived from an internal class called _String_val which, by unlucky coincidence, just happens to be a one-parameter template. So Cont is deduced to be _String_val , but then the instantiation fails as _String_val does not have a method named size (that method is implemented by basic_string itself).

Despite your claim to the contrary, I get a similar error when using std::vector in place of std::basic_string - for the exact same reason.

Now, there's no reason to make Cont a template template parameter (and a good reason not to - it won't work). Make it a plain type parameter, or else have the function take a pair of iterators. Something along these lines:

template<typename Cont>
void writeContainer(Stream& stream, const Cont& outValue);

// or

template<typename Iter>
void writeRange(Stream& stream, Iter first, Iter last);