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I am trying to initialize a constexpr reference with no success. I tried

#include <iostream> constexpr int& f(int& x) // can define functions returning constexpr references { return x; } int main() { constexpr int x{20}; constexpr const int& z = x; // error here }

but I'm getting a compile time error

error: constexpr variable 'z' must be initialized by a constant expression

Dropping the const results in

error: binding of reference to type 'int' to a value of type 'const int' drops qualifiers

even though I had the feeling that constexpr automatically implies const for variable declarations.

So my questions are:

  1. Are constexpr references ever useful? (i.e., "better" than const references)
  2. If yes, how can I effectively define them?

PS: I've seen a couple of questions related to mine, such as Which values can be assigned to a `constexpr` reference? , but I don't think they address my questions.

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3 Answers 3

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  1. Are constexpr references ever useful? (i.e., "better" than const references)

They are guaranteed to be initiailized before the program starts, whereas a reference to const can be initialized during dynamic initialization, after the program starts running.

  1. If yes, how can I effectively define them?

A constexpr reference has to bind to a global, not a local variable (or more formally, it has to bind to something with static storage duration).

A reference is conceptually equivalent to taking the address of the variable, and the address of a local variable is not a constant (even in main which can only be called once and so its local variables are only initialized once).

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11 Comments

They need to bind to something with static storage duration, which can be a local static or a global.
constexpr affects the reference, not the type it binds to, so to bind to a const int you still need a const int&
@antred [basic.start.main]/3: "The function main shall not be used within a program. The linkage of main is implementation-defined. A program that defines main as deleted or that declares main to be inline, static, or constexpr is illformed. The name main is not otherwise reserved."
@antred I'm no C expert, but a quick look at N1570 shows no similar restrictions. 5.1.2.2.3/1 Program Termination does say "If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument..." [emphasis added]. This would seem to imply that non-initial calls to main are compliant.
@T.C. "which can be a local static or a global" or a temporary whose lifetime is extended to static storage duration: static constexpr int const& x = 42; is fine. That's also why static constexpr auto x = {1,2}; is accepted by clang++, but constexpr auto x = {1,2}; is not (at block scope). (This implies that even static constexpr const int& z = +x; in the OP's code is legal.)
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So the problem is that a constexpr reference needs to bind to an object with static storage duration, which is covered in the draft C++11 standard: N3337 section 5.19 [expr.const] (emphasis mine):

A reference constant expression is an lvalue core constant expression that designates an object with static storage duration or a function

The draft C++14 standard: N3936 changes the wording:

A constant expression is either a glvalue core constant expression whose value refers to an object with static storage duration or to a function, or a prvalue core constant expression whose value is an object where, for that object and its subobjects:

  • each non-static data member of reference type refers to an object with static storage duration or to a function, and
  • if the object or subobject is of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object (5.7), the address of a function, or a null pointer value.

So changing the declaration of x like so would work:

constexpr static int x{20};
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Like T.C. says, the initializer needs to be an object with static storage duration.

N4140/§5.19/4 A constant expression is either a glvalue core constant expression whose value refers to an object with static storage duration [...]

N4140/§7.1.5/9 A constexpr specifier used in an object declaration declares the object as const. Such an object shall have literal type and shall be initialized. [...] Otherwise, or if a constexpr specifier is used in a reference declaration, every full-expression that appears in its initializer shall be a constant expression.

In N3337, the wording is different.

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