C++ type/value mismatch at argument 1 in template parameter

Question

Ok so I have this code below and when I execute it I get the following error:

type/value mismatch at argument 1 in template parameter list for ‘template<class _Tp, class _Alloc> class std::vector’
     vector<s> v;

template <class T>
class A {
public:
    struct s{T x;};
};

template <class T>
class B: public A<T> {
public:
    using A<T>::s;
    vector<s> v;
};

Can someone please explain the problem.

See also https://stackoverflow.com/questions/610245/where-and-why-do-i-have-to-put-the-template-and-typename-keywords. — melpomene, Sep 16 '19 at 07:33

score 7 · Accepted Answer · edited Sep 16 '19 at 07:13

7

The issue is that the compiler doesn't know whether s is a type or a value. This is the case where you add typename or template, but neither of those worked when I tested. Using the full type directly in the vector does work however:

template <class T>
class B: public A<T> {
public:
    using A<T>::s;
    std::vector<typename A<T>::s> v;
};

(Edit) Kept playing with it because why not, and of course typename was needed in the one place I didn't trying it: the using line. The following code also works.

template <class T>
class B: public A<T> {
public:
    using typename A<T>::s;
    std::vector<s> v;
};

edited Sep 16 '19 at 07:13

melpomene

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answered Mar 17 '18 at 02:22

Stephen Newell

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this typename requirement always stumps me – Abhinav Gauniyal Mar 17 '18 at 02:32
Cool, it works. Interesting, when I do `using A::someobject` I don't need the typename. Looks like the typename is required when you use some defined structure. – Keloo Mar 17 '18 at 02:33

C++ type/value mismatch at argument 1 in template parameter

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