| johnsfine |
04-07-2012 02:37 PM |
C++ template function default type and value?
I want to define a function in which the type of one of the inputs is a template parameter (inferred if the input is given) but that input is also optional with a default value.
Is there any correct syntax for that in C++?
Incorrect syntax demonstrating what I want to do is:
Code:
struct nothing {};
template< class T >
int test_template(int x, T const& y = nothing()) {
return x;
}
int test() {
int i = test_template(1);
int j = test_template(2,3);
int k = test_template<nothing>(4);
return i+j;
}
g++ complains about test_template(1) because it can't infer the type T. Other compilers are OK with that line but complain about test_template(2,3), which g++ likes but the other compilers complain the default value is incompatible with the inferred type, which is true but a really annoying check since the default value isn't used in the case where it isn't compatible. All the compiler are OK with test_template<nothing>(4) but that is only there for illustration. It isn't actually helpful.
Logically the code might be this instead, but no compiler likes this either:
Code:
struct nothing {};
template< class T = nothing >
int test_template(int x, T const& y = T()) {
return x;
}
int test() {
int i = test_template(1);
int j = test_template(2,3);
int k = test_template<nothing>(4);
return i+j;
}
In the real use, the function replacing test has a lot of templated parameters before the one I want to default and does a lot of complicated operations on them and needs a compile time check (which I know how to code) on whether the defaulted parameter has type nothing to remove code that should only be present if that parameter was provided.
The best, but ugly, solution I can find is equivalent to:
Code:
struct nothing {};
template< class T>
int test_template(int x, T const& y) {
return x;
}
int test_template(int x) {
return test_template(x, nothing());
}
int test() {
int i = test_template(1);
int j = test_template(2,3);
return i+j;
}
Anyone have any better ideas? |
