C++: Template Function taking rvalue reference overloads non-template function

Question

Trying to run a non-generic function but it is overloaded by a template function.

The problem is that they take rvalue references as arguments.

Here's what it kinda looks like:

#include <iostream> using namespace std; template <typename T> void bar(T &b) { cout << "bar <template> called\n" ; } template <typename T> void foo(T &&b) { cout << "foo <template> called\n" ; } void bar(string &b) { cout << "bar <string> called\n" ; } void foo(string &&b) { cout << "foo <string> called\n" ; } int main() { string msg_a = "hello a"; string msg_b = "hello b"; int a = 1; int b = 2; bar(a); bar(msg_a); bar(b); bar(msg_b); cout << "\n"; foo(a); foo(msg_a); // <-- I want this to call the 'void foo(string &&b)' but it doesn't foo(b); foo(msg_b); // <-- I want this to call the 'void foo(string &&b)' but it doesn't return (0); }

Output: bar <template> called bar <string> called bar <template> called bar <string> called foo <template> called foo <template> called foo <template> called foo <template> called

When I call foo() with a type string I would like it to call the void foo(string &&b) function but instead it calls the template <typename T> void foo(T &&b) function.

Whereas as you can see with the functions that take an lvalue reference this is not a problem and the priority is held as normal.

Does anyone know a fix or work around for this ?

Vittorio Romeo · Accepted Answer · 2019-08-10 00:20:13Z

4

foo(msg_a) cannot ever call void foo(string &&b) as that particular overload only accepts rvalues of type string, while msg_a is an lvalue expression. The only viable fallback is therefore template <typename T> void foo(T &&b), which accepts a forwarding reference. Forwarding references bind to both lvalues and rvalues.

If you invoke foo with an rvalue (e.g. foo(std::string{})), then it will call the aforementioned overload.

answered Aug 10, 2019 at 0:20

Vittorio Romeo

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4 Comments

AymenTM Over a year ago

In other words i need to make foo(std::string &b) instead of foo(std::string &&b) and forget trying to get rvalues, basically forcing me to strictly only get lvalues ?

Vittorio Romeo Over a year ago

@AymenTM: possibly. I am not sure what your real use case is and what you're trying to achieve. But technically that would work.

AymenTM Over a year ago

Trying to have a template function do something for all types except for std::strings, I want it to do something specific. Ik about specializations but isn't this like the most specific ?

Vittorio Romeo Over a year ago

@AymenTM: you could use if constexpr + a type trait instead of relying on overloading.

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