c ++模板参数中的常量表达式

Question

I have a template, that takes a char argument like:

A<'T'>

I am storing my T in a variable like:

const char ch = str[0]; //str is a string from my program

constexpr char ch = str[0]; // this doesnt work either for me

I am trying to achieve this:

A<ch>();

I am using gcc 4.7 and have dabbled with constexpr but I havent been able to get that work

Any idea of a way to get this to work? Any help is appreciated

Answer 1

This can only work if everything is a constant expression:

constexpr char str[] = "Hello World";
constexpr char ch = str[0];
A<ch> x;

Answer 2

If the contents of str are defined at runtime, then there is no way to achieve that. The compiler requires your template value to be set during compilation.

That is why this is valid:

A<'a'>();

Since 'a' is a constant value, known during compilation. But this:

void foo(const std::string &value) {
    A<value[0]> t;
}

Is not, since value[0] , despite being a constant value, is not known during compilation.