c++ - constexpr function parameters as template arguments

Question

I am playing around with some toy code using c++11 to figure out a bit more about how things work. During this I came across the following issue that simplifies down to:

template <int x, int y>
class add {
public:
    static constexpr int ret = x + y;
};

constexpr int addFunc(const int x, const int y) {
    return add<x,y>::ret;
}

int main() {
    const int x = 1;
    const int y = 2;
    cout << add<x,y>::ret << endl; // Works
    cout << addFunc(1,2) << endl;  // Compiler error
    return 0;
}

I'm using GCC 4.8.1 and the output is:
'x' is not a constant expression in template argument for type 'int'
'y' is not a constant expression in template argument for type 'int'

What exactly is the difference between the two ways I am trying to calculate add::ret? Both of these values should be available at compile time.

Answer 1

If your purpose is just to shorten code a bit, in C++14 you can create variable template:

template <int x, int y>
constexpr int addVar = x + y;

cout << addVar<5, 6> << endl; // Works with clang 3.5, fails on GCC 4.9.1

GCC 5 will also support this.