编译错误，以迭代器作为补充的C ++模板化函数

Question

I thought I was starting to get the hang of C++ ....

Then I wrote what I thought we are very simple templated function and all of a sudden it seems like nothing makes sense again. The compiler doesn't even seem to like the fact that I have defined a templated function, which seems a bit crazy. It is single unit of compilation so I am not sure what it would be complaining about.

#include <vector>
#include <iostream>

typedef std::vector<int> int_vec_type;

template <typename Type>
bool print_vec(Type::const_iterator itr, const Type::const_iterator end)
{
    for (; itr != end; ++itr) {
        std::cout << *itr << std::endl;
    }

    return true;
}

int
main()
{
    int_vec_type ivec;

    ivec.push_back(0);
    ivec.push_back(1);
    ivec.push_back(2);

    print_vec(ivec.begin(), ivec.end());

    return 0;
}

these are the compilation errors:

tia.cc:7:22: error: template declaration of 'bool print_vec'

tia.cc:7:37: error: expected ')' before 'itr'

tia.cc:7:42: error: expected primary-expression before 'const'

tia.cc: In function 'int main()':

tia.cc:25:39: error: 'print_vec' was not declared in this scope

thanks in advance.

Answer 1

The type of the container is not deducible from the type of the iterator. You can simply transform the template into:

template <typename Iterator>
bool print_vec(Iterator itr, const Iterator end)
{
    for (; itr != end; ++itr) {
        std::cout << *itr << std::endl;
    }

    return true;
}

Answer 2

First problem: You have not used typename . Wherever a type depends on a template argument, you must prefix the type with typename .

Secondly, the compiler cannot infer type . It can only see the type of ivec.begin() and has no idea about any other types that might have it as a typedef. You can only take the const_iterator directly- you can't take a T::const_iterator - without passing T explicitly, anyway.

Answer 3

The best solution would be to template in terms of iterator types, since the container's type cannot be deduced from the function arguments:

template <typename Iterator>
bool print_vec(Iterator itr, Iterator end) { .... }