Question

I’m learning how to use class templates. I have read a number of examples, but I still have some problems.

I have the following template class in my header foo.h:

template<typename T>
class Foo
{
 public:
    bool addKey(const std::string& key);
    bool addValue(const std::string& key, const T& value);

private:
    std::map<std::string, T> mapping;
};

This is the implementation file foo.cpp:

template <typename T>
bool Foo<T>::addKey(const string& key)
{
    if (key.empty()) 
        return false;

    pair<map<string, T>::iterator, bool> response; // to store the pair returned by insert()
    response = mapping.insert(pair<string, T>(key, T()));

    return response.second;
}

The followings are the compilation errors (g++ within Kdevelop)

error: type/value mismatch at argument 1 in template parameter list for ‘template<class _T1, class _T2> struct std::pair’
error:   expected a type, got ‘std::map<std::basic_string<char>, T>::iterator’
error: invalid type in declaration before ‘;’ token
error: request for member ‘second’ in ‘response’, which is of non-class type ‘int’

So it looks like std::pair cannot deal with T type?

If I don’t save the std::pair returned by insert(), compilation works fine.

Answer 1

In this case iterator is a dependent name, you should qualify it with the typename keyword:

 pair<typename map<string, T>::iterator, bool>

See this question for more details.