Why doesn't this compile:
Could there be a problem with a string as a return type?
constexpr std::string fnc()
{
return std::string("Yaba");
}
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1 Answer
The constructor of std::string that takes a pointer to char is not constexpr. In constexpr functions you can only use functions that are constexpr.
answered Oct 15, 2011 at 16:22
R. Martinho Fernandes
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9 Comments
smallB
Thanks, didn't know that before.
Jon Purdy
+1 and the reason that that constructor can’t be
constexpr is that it has side-effects (namely allocation) that can’t be carried out at compile time.Jon Purdy
@smallB: A constant-expression constructor must be 1. declared
constexpr 2. have a member-initializer part involving only potential constant-expressions and 3. have an empty body. It seems to me that std::string has to violate #2.Jon Purdy
@smallB: Okay. I never said anything to the contrary.
Luc Danton
@smallB You can't have a
constexpr constructor for std::string that takes arbitrary length const char* and be safe. While it's true that string literals are constant expressions, there are plenty of things of type const char* that are not constant expressions. The c_str member of std::string comes to mind. Overloading on constexpr is not available to distinguish between the two. |