Question

What is the effect of the const keyword in this template?

template <class T, int const ROWNUM, int const COLNUM> 
class Matrix

Does it mean that this template only accept a const as parameter? If so, is there a way to pass a variable as the COLNUM and ROWNUM?

(when I try to pass a variable as the COLNUM for the template, it gives an error: “IntelliSense: expression must have a constant value”)

Answer 1

It’s ignored:

[C++11: 14.1/4]: A non-type template-parameter shall have one of the following (optionally cv-qualified) types:

• integral or enumeration type,
• pointer to object or pointer to function,
• lvalue reference to object or lvalue reference to function,
• pointer to member,
• std::nullptr_t.

[C++11: 14.1/5]: [ Note: Other types are disallowed either explicitly below or implicitly by the rules governing the form of template-arguments (14.3). —end note ] The top-level cv-qualifiers on the template-parameter are ignored when determining its type.

The same wording is present at the same location in C++03.

This is partially because template arguments must be known at compile-time anyway. So, whether you have the const there or not, you may not pass some variable value:

template <int N>
void f()
{
    N = 42;
}

template <int const N>
void g()
{
    N = 42;
}

int main()
{
    f<0>();
    g<0>();

    static const int h = 1;
    f<h>();
    g<h>();
}

prog.cpp: In function ‘void f() [with int N = 0]’:
prog.cpp:15: instantiated from here
prog.cpp:4: error: lvalue required as left operand of assignment
prog.cpp: In function ‘void g() [with int N = 0]’:
prog.cpp:16: instantiated from here
prog.cpp:10: error: lvalue required as left operand of assignment
prog.cpp: In function ‘void f() [with int N = 1]’:
prog.cpp:19: instantiated from here
prog.cpp:4: error: lvalue required as left operand of assignment
prog.cpp: In function ‘void g() [with int N = 1]’:
prog.cpp:20: instantiated from here
prog.cpp:10: error: lvalue required as left operand of assignment