I'm getting a strange compiler error when trying to create constexpr
std::string
and std::vector
objects:
#include <vector>
#include <string>
int main()
{
constexpr std::string cs{ "hello" };
constexpr std::vector cv{ 1, 2, 3 };
return 0;
}
The compiler complains that "the expression must have a constant value":
Am I missing something? I am using the latest Microsoft Visual Studio 2019 version: 16.11.4, and the reference (https://en.cppreference.com/w/cpp/compiler_support) states that constexpr
strings and vectors are supported by this compiler version:
I have also tried the constexpr std::array
, which does work. Could the issue have anything to do with the dynamic memory allocation associated with vectors?
CodePudding user response:
Your program is actually ill-formed, though the error may be hard to understand. constexpr
allocation support in C 20 is limited - you can only have transient allocation. That is, the allocation has to be completely deallocated by the end of constant evaluation.
So you cannot write this:
int main() {
constexpr std::vector<int> v = {1, 2, 3};
}
Because v
's allocation persists - it is non-transient. That's what the error is telling you:
<source>(6): error C2131: expression did not evaluate to a constant
<source>(6): note: (sub-)object points to memory which was heap allocated during constant evaluation
v
can't be constant because it's still holding on to heap allocation, and it's not allowed to do so.
But you can write this:
constexpr int f() {
std::vector<int> v = {1, 2, 3};
return v.size();
}
static_assert(f() == 3);
Here, v
's allocation is transient - the memory is deallocated when f()
returns. But we can still use a std::vector
during constexpr
time.