Why are the first two calls to doSomething
OK by the compiler, but using two elements in the list causes an ambiguous call?
#include <vector> #include <string> void doSomething(const std::vector<std::string>& data) {} void doSomething(const std::vector<int>& data) {} int main(int argc, char *argv[]) { doSomething({"hello"}); // OK doSomething({"hello", "stack", "overflow"}); // OK doSomething({"hello", "stack"}); // C2668 'doSomething': ambiguous call return 0; }
What is happening here is that in the two element initializer list both of the string literals can be implicitly converted to const char*
since their type is const char[N]
. Now std::vector
has a constructor that takes two iterators which the pointers qualify for. Because of that the initializer_list
constructor of the std::vector<std::string>
is conflicting with the iterator range constructor of std::vector<int>
.
If we change the code to instead be
doSomething({"hello"s, "stack"s});
Then the elements of the initializer list are now std::string
s so there is no ambiguity.
Both the one-argument and three-argument lists can only match std::vector<std::string>
's std::initializer_list
constructor. However, the two-argument list matches one of the constructors from std::vector<int>
:
template <class InputIt> vector(InputIt first, InputIt last, Allocator const &alloc = Allocator());
Indeed, a char const *
can be incremented, and dereferenced to get a char
that is implicitly convertible to an int
.
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