声明采用模板参数的函数

Question

I want to define a helper function that takes a template parameter. I have tried making a templated function for this, but it does not compile. Any idea what I'm doing wrong? Here's the code I tried.

// vectors are great, but lack a find method. Implement one as a helper.
template<class T> bool vec_find(vector<T> &v, T obj)
{
    vector<T>::iterator s;
    for (s = v.begin(); s < v.end(); s++)
    {
        if (*s == obj)
        {
            return true;
        }
    }
    return false;
}

Answer 1

Presumably, your compiler told you what the problem was. Mine said:

test.cpp:7:5: error: need ‘typename’ before ‘std::vector<T>::iterator’ because ‘std::vector<T>’ is a dependent scope

So to fix it, add typename before vector<T>::iterator :

typename vector<T>::iterator s;
^^^^^^^^

In general, you need that whenever the scope of a type name depends on a template parameter; until the template is instantiated, the compiler doesn't know how vector<T> will be defined, and so needs to be told that a name scoped inside it refers to a type rather than something else.

However, there is a good reason why vector doesn't have a find method: the C++ library separates containers from the algorithms that act on them, so that any algorithm can act on any suitable sequence. You want to use std::find for this:

return std::find(v.begin(), v.end(), obj) != v.end();