CODEWITHSUNDEEP
c++functionc++11constexpr
SLN
SLN

Reputation: 5092

constexpr qualifier added on a non constexpr function does not trigger any warning

Seems the compiler ignores the constexpr qualifier when it is added to a non-constexpr function. Why is that?

The following code compiles fine and runs.

#include <iostream>
#include <string>
using std::string; using std::cout; using std::endl;

constexpr bool is_shorter(const string &lft, const string &rht) // this is not a constexpr function
{
    return lft.size() < rht.size();
}

int main()
{
    bool restul =  is_shorter("Hello", "World!");
    return 0;
}

Upvotes: 0

Views: 83

Answers (1)

NathanOliver
NathanOliver

Reputation: 180945

The reason this happens is because the standard allows it to do so. [dcl.constexpr]/5 states

For a constexpr function or constexpr constructor that is neither defaulted nor a template, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression (8.20), or, for a constructor, a constant initializer for some object (6.6.2), the program is ill-formed, no diagnostic required.

So, since the function can never be a core constant expression the behavior is undefined and the compiler is not required to notify you.

Upvotes: 3

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