Function overload for string literals lvalue and rvalue reference

Question

The function test below is overloaded for lvalue empty strings, lvalue non-empty strings and rvalue strings. I tried to compile with Clang and GCC but in both case I do not have the result I expected.

#include <iostream>

void test(const char (&)[1]){ std::cout << __PRETTY_FUNCTION__ << std::endl; }

template <unsigned long int N>
void test(const char (&)[N]){ std::cout << __PRETTY_FUNCTION__ << std::endl; }

void test(char*&&){ std::cout << __PRETTY_FUNCTION__ << std::endl; }

int main(){
    char str1[] = "";
    char str2[] = "test";
    test("");
    test("test");
    test(str1);
    test(str2);
}

Output with clang version 6.0.0-1ubuntu2:

clang++ test.cpp -o test.out && ./test.out
void test(const char (&)[1])
void test(const char (&)[N]) [N = 5]
void test(char *&&)
void test(char *&&)

Output with g++ (MinGW.org GCC-8.2.0-3):

g++ test.cpp -o test.exe && test.exe
test.cpp: In function 'int main()':
test.cpp:15:11: error: call of overloaded 'test(char [1])' is ambiguous
  test(str1);
           ^
test.cpp:3:6: note: candidate: 'void test(const char (&)[1])'
 void test(const char (&)[1]){ std::cout << __PRETTY_FUNCTION__ << std::endl; }
      ^~~~
test.cpp:6:6: note: candidate: 'void test(const char (&)[N]) [with long unsigned int N = 1]'
 void test(const char (&)[N]){ std::cout << __PRETTY_FUNCTION__ << std::endl; }
      ^~~~
test.cpp:8:6: note: candidate: 'void test(char*&&)'
 void test(char*&&){ std::cout << __PRETTY_FUNCTION__ << std::endl; }
      ^~~~

My questions are:

Which compiler is correct?
With Clang, why test(str1) and test(str2) choose the rvalue overload while they are lvalues?
With GCC, why the call test(str1) is ambiguous?
Is there a standard rule for this situation?
How to fix the two last calls?

Thank you.

songyuanyao · Accepted Answer · 2019-09-28T08:25:48.553

Which compiler is correct ?

GCC is correct.

With clang, why str1 and str2 choose the rvalue overload while they are lvalues ?

Clang is wrong on test(str1);, it should be ambiguous. For test(str2);, str2 could convert to pointer implicitly, i.e. the array-to-pointer decay. The converted char* is an rvalue. For the same reason as #3, the implicit conversion sequences have the same ranking, then non-template function is prefered; test(char*&&) is selected.

With gcc, why call with str1 is ambiguous ?

For test(const char (&)[1]) to be called, qualification conversion from char[1] to const char[1] is required; for test(char*&&) to be called, array-to-pointer conversion is required. Both are qualified as exact match and have the same ranking.

Is there a standard rule for this situation ?

See the ranking of implicit conversion sequences in overload resolution, and implicit conversions.

How to fix the two last calls ?

It depends on your intent.

Darklighter · Answer 2 · 2019-09-27T18:59:16.027

String literals are not rvalues. (→)

How to fix the two last calls?

You can disambiguate everything with template specializations:

#include <iostream>

template<typename C, std::size_t N>
void test(const C (&)[N]) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
template<typename C>
void test(const C (&)[1]) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
template<typename C, std::size_t N>
void test(const C (&&)[N]) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
template<typename C>
void test(const C (&&)[1]) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
template<typename C, std::size_t N>
void test(C (&)[N]) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
template<typename C>
void test(C (&)[1]) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
template<typename C, std::size_t N>
void test(C (&&)[N]) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
template<typename C>
void test(C (&&)[1]) { std::cout << __PRETTY_FUNCTION__ << std::endl; }

int main(){
    char str1[] = "";
    char str2[] = "test";
    test("");
    test("test");
    test(str1);
    test(str2);
    test(std::move(str1));
    test(std::move(str2));
    const char str3[] = "";
    const char str4[] = "test";
    test(std::move(str3));
    test(std::move(str4));
}

gives

void test(const C (&)[1]) [with C = char]
void test(const C (&)[N]) [with C = char; long unsigned int N = 5]
void test(C (&)[1]) [with C = char]
void test(C (&)[N]) [with C = char; long unsigned int N = 5]
void test(C (&&)[1]) [with C = char]
void test(C (&&)[N]) [with C = char; long unsigned int N = 5]
void test(const C (&&)[1]) [with C = char]
void test(const C (&&)[N]) [with C = char; long unsigned int N = 5]

score 1 · Answer 3 · answered Sep 30 '19 at 10:30

Thank you @songyuanyao for your answer, I understand now why test(char*&&) is chosen in the two last cases. I was able to remove ambiguity with template specialization on first overload thanks to @Darklighter answer too.

So I solved my problem such as below :

#include <iostream>

template <unsigned long int N>
void test(const char (&)[N]){
    std::cout << __PRETTY_FUNCTION__ << " //non-empty literal" << std::endl;
}

template <>
void test(const char (&)[1]){
    std::cout << __PRETTY_FUNCTION__ << " //empty literal" << std::endl;
}

void test(char*&&){
    std::cout << __PRETTY_FUNCTION__ << " //string variable" << std::endl;
}

int main(){
    char str1[] = "";
    char str2[] = "test";
    test("");
    test("test");
    test(str1);
    test(str2);
}

Output :

clang++ test.cpp -o test.out && ./test.out
void test(const char (&)[1]) //empty literal
void test(const char (&)[N]) [N = 5] //non-empty literal
void test(char *&&) //string variable
void test(char *&&) //string variable

g++ test.cpp -o test.exe && test.exe
void test(const char (&)[N]) [with long unsigned int N = 1] //empty literal
void test(const char (&)[N]) [with long unsigned int N = 5] //non-empty literal
void test(char*&&) //string variable
void test(char*&&) //string variable

Function overload for string literals lvalue and rvalue reference

3 Answers3