Cannot refer to a template name nested in a template parameter

Question

I have the following code:

template <typename Provider>
inline void use()
{
    typedef Provider::Data<int> D;
}

Where I'm basically trying to use a template class member 'Data' of some 'Provider' class, applied to 'int', but I get the following errors:

util.cpp:5: error: expected init-declarator before '<' token
util.cpp:5: error: expected `,' or `;' before '<' token

I'm using GCC 4.3.3 on a Solaris System.

possible duplicate of [C++ template member function of template class called from template function](http://stackoverflow.com/questions/1840253/c-template-member-function-of-template-class-called-from-template-function) — Kirill V. Lyadvinsky, May 18 '10 at 19:38
@Kirill: I think this one is unique in that it requires both `typename` and `template`. (I struggled for a while with how to explain this using the same code and I'm not satisfied.) — sbi, May 18 '10 at 19:43
@sbi, yes, this is not exact the same, but very similar question. — Kirill V. Lyadvinsky, May 18 '10 at 19:47

sbi · Accepted Answer · 2010-05-18T19:34:10.990

typedef typename Provider::template Data<int> D;

The problem is that, when the compilers parses use() for the first time, it doesn't know Provider, so it doesn't know what Provider::Data refers to. It could be a static data member, the name of a member function or something else. That's why you have to put the typename in.
The additional template is necessary whenever the nested name is the name of a template. If it was something else, then Data < ... could be a comparison.

score 8 · Answer 2 · answered May 18 '10 at 19:30

8

You need a typename and a template:

template <typename Provider>
inline void use()
{
    typedef typename Provider::template Data<int> D;
}

answered May 18 '10 at 19:30

James McNellis

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Cannot refer to a template name nested in a template parameter

2 Answers2