A function `getVar` that gets the value of a static variable inside a class (template argument) and returns a default value, if that static variable is not present.

static_var_default.cpp

#include <iostream>

using VarType = int;
const VarType DEFAULT_VAR = 0;

struct doesHaveVar {
    static const VarType var = 1;
};

struct doesNotHaveVar {
};

// "..."" vs "int" lowers this function's priority, so if T::var is present, the other function will be chosen over this
template <class T>
constexpr VarType getVarImpl(const T* t, ...) {
    return DEFAULT_VAR;
}

// T::var is a substitution failure, so if var is not present, this will be ignored
template <class T>
constexpr typename std::enable_if<!std::is_void<decltype(T::var)>::value, VarType>::type
getVarImpl(const T* t, int) {
    return T::var;
}

// We need the indirection, so T has to be deduced for getVarImpl
template <typename T>
constexpr VarType getVar() {
    return getVarImpl(static_cast<T*>(nullptr), 0);
}

int main(int argc, char** argv) {
    std::cout << getVar<doesHaveVar>() << std::endl;
    std::cout << getVar<doesNotHaveVar>() << std::endl;
}

Output is:

1
0