how to provide a swap function for my class?
c++, stl, swap
Solution
- is the proper use of `swap`. Write it this way when you write "library" code and want to enable ADL (argument-dependent lookup) on `swap`. Also, this has nothing to do with SFINAE.
// some algorithm in your code
template<class T>
void foo(T& lhs, T& rhs) {
using std::swap; // enable 'std::swap' to be found
// if no other 'swap' is found through ADL
// some code ...
swap(lhs, rhs); // unqualified call, uses ADL and finds a fitting 'swap'
// or falls back on 'std::swap'
// more code ...
}
- Here is the proper way to provide a `swap` function for your class:
namespace Foo {
class Bar{}; // dummy
void swap(Bar& lhs, Bar& rhs) {
// ...
}
}
If `swap` is now used as shown in 1), your function will be found. Also, you may make that function a friend if you absolutely need to, or provide a member `swap` that is called by the free function:
// version 1
class Bar{
public:
friend void swap(Bar& lhs, Bar& rhs) {
// ....
}
};
// version 2
class Bar{
public:
void swap(Bar& other) {
// ...
}
};
void swap(Bar& lhs, Bar& rhs) {
lhs.swap(rhs);
}
...
- You mean an explicit specialization. Partial is still something else and also not possible for functions, only structs / classes. As such, since you can't specialize `std::swap` for template classes, you have to provide a free function in your namespace. Not a bad thing, if I may say so. Now, an explicit specialization is also possible, but generally you do not want to specialize a function template:
namespace std
{ // only allowed to extend namespace std with specializations
template<> // specialization
void swap<Bar>(Bar& lhs, Bar& rhs) noexcept {
// ...
}
}
- No, as 1) is distinct from 2) and 3). Also, having both 2) and 3) will lead to always having 2) picked, because it fits better.
Problem
What is the proper way to enable my `swap` in STL algorithms? 1) Member `swap`. Does `std::swap` use SFINAE trick to use the member `swap`. 2) Free standing `swap` in the same namespace. 3) Partial specialization of `std::swap`. 4) All of the above. Thank you. EDIT: Looks like I didn't word my question clearly. Basically, I have a template class and I need STL algos to use the (efficient) swap method I wrote for that class.