c++ - Difference between string literal and constexpr array of char

Question

I have been wondering if there is any difference between what is being pointed by ptrToArray and ptrToLiteral in the following example:

constexpr char constExprArray[] = "hello";
const char* ptrToArray = constExprArray;

const char* ptrToLiteral = "hello";

• Is my understanding that constExprArray and the two "hello" literals are all compile time constant lvalues correct?
• If so, is there any difference in how they are stored in the executable file, or is it purely compiler implementation or platform specific?
• Are they treated differently at runtime behind the scenes?
• Anything else to know about?

Answer 1

A string literal and a constexpr array of char are almost identical. A pointer to either is an address constant expression. An lvalue-to-rvalue conversion is permitted on their elements in a constant expression. They both have static storage duration. The only difference that I know of is that a string literal can initialize an array whereas a constexpr array cannot:

constexpr char a[] = "hello";

constexpr char b[] = a; // ill-formed
constexpr char b[] = "hello"; // ok

To get around this you can wrap the array in a class of literal type. We are currently looking at standardizing such a wrapper that will be called std::string_literal or similar, but for now you will have to do this by hand.