I've written the following code intended to allow my class, mytype
, to select at compile-time whether to use a C-style array or a C++ STL array, like so:
#include<array>
#include<cassert>
template<bool IsTrue, typename IfTrue, typename IfFalse>
struct choose;
template<typename IfTrue, typename IfFalse>
struct choose<true, IfTrue, IfFalse> {
typedef IfTrue type;
};
template<typename IfTrue, typename IfFalse>
struct choose<false, IfTrue, IfFalse> {
typedef IfFalse type;
};
template<bool ArrayIsRaw>
struct mytype {
typedef typename choose<ArrayIsRaw, int[50], std::array<int, 50>>::type array_t;
array_t data{};
};
int main() {
mytype<true> raw_version;
mytype<false> stl_version;
raw_version.data[5] = 15;
stl_version.data[15] = 5;
raw_version.data[10] = stl_version.data[15];
assert(raw_version.data[10] == 5);
return 0;
}
This works just fine. However, I want to add an equality operator to this class that is agnostic of the underlying types involved: basically, I want raw_version == stl_version
to be valid, compileable code which will return true
if each element is identical.
But when I add the following code to my class definition:
template<bool Raw1, bool Raw2>
friend bool operator==(mytype<Raw1> const& a, mytype<Raw2> const& b) {
for(size_t i = 0; i < 50; i++) if(a.data[i] != b.data[i]) return false;
return true;
}
I get the following error:
prog.cpp: In instantiation of ‘struct mytype<false>’:
prog.cpp:32:16: required from here
prog.cpp:24:14: error: redefinition of ‘template<bool Raw1, bool Raw2> bool operator==(const mytype<Raw1>&, const mytype<Raw2>&)’
friend bool operator==(mytype<Raw1> const& a, mytype<Raw2> const& b) {
^~~~~~~~
prog.cpp:24:14: note: ‘template<bool Raw1, bool Raw2> bool operator==(const mytype<Raw1>&, const mytype<Raw2>&)’ previously defined here
What do I need to do to fix this error?
By making both arguments to operator==
templated, you are redefining this exact function template for mytype<true>
and mytype<false>
. Simply remove the templating from the first (or second, but not both) argument to make this work:
template<bool Raw2>
friend bool operator==(mytype const& a, mytype<Raw2> const& b) {
// ...
}
It also appears that your choose
is just an implementation of std::conditional
and you could instead have
using array_t = typename std::conditional<ArrayIsRaw, int[50], std::array<int, 50>>::type;
or for c++14
using array_t = std::conditional_t<ArrayIsRaw, int[50], std::array<int, 50>>;
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