C++: How to use unnamed template parameters in class members?

Question

I am creating a simple Matrix class. I am trying to add an unnamed template parameter to make sure it is used with integral types

#include <string>
#include <vector>
#include <boost/utility/enable_if.hpp>
#include <boost/type_traits/is_scalar.hpp>

template <typename T, typename = typename boost::enable_if<boost::is_scalar<T> >::type>
class Matrix
{
    public:
        Matrix(const size_t nrow, const size_t ncol);

    private:
        const size_t nrow_;
        const size_t ncol_;
        std::vector<std::string> rownames_;
        std::vector<std::string> colnames_;
        std::vector<T> data_;
};

I would like to define the constructor outside the class definition

template <typename T,typename>
inline Matrix<T>::Matrix(size_t nrow, size_t ncol)
    : nrow_(nrow),
    ncol_(ncol),
    rownames_(nrow_),
    colnames_(ncol_),
    data_(nrow_*ncol)
{};

g++ returns the following error

Matrix.hh:25:50: error: invalid use of incomplete type ‘class Matrix<T>’
 inline Matrix<T>::Matrix(size_t nrow, size_t ncol)

Do you know how to solve this issue?

Thanks in advance.

No, in the header, just after the comma ending the class definition. — Florian Plaza Oñate, Feb 26 '16 at 12:35

score 10 · Accepted Answer · edited May 23 '17 at 11:59

Template parameter names are "local" to each template declaration. Nothing prevents you from assigning a name. Which you indeed must do if you need to refer to that parameter later (such as using it as an argument in the template-id of the class).

So, while you have this in the class definition:

template <typename T, typename = typename boost::enable_if<boost::is_scalar<T> >::type>
class Matrix
{
    public:
        Matrix(const size_t nrow, const size_t ncol);

    private:
        const size_t nrow_;
        const size_t ncol_;
        std::vector<std::string> rownames_;
        std::vector<std::string> colnames_;
        std::vector<T> data_;
};

You can define it outside the class e.g. like this:

template <typename AnotherName,typename INeedTheName>
inline Matrix<AnotherName, INeedTheName>::Matrix(size_t nrow, size_t ncol)
    : nrow_(nrow),
    ncol_(ncol),
    rownames_(nrow_),
    colnames_(ncol_),
    data_(nrow_*ncol)
{};

Just do not forget that under common circumstances, templates can only be defined in header files.

C++: How to use unnamed template parameters in class members?

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