About func(const int&) and func(const int)

Question

#include <iostream>

class Account {
public:
    static double GetCircumference(const double &dR) { return 2 * dR * 3.1415926; }
    static constexpr double cd = 3.0;
};

// constexpr double Account::cd;

int main()
{
    std::cout << Account::GetCircumference(Account::cd) << std::endl;
}

The code is wrong unless I remove the "//". But if I only change (const double &dR) to (const double dR), it becomes ok too. Why?

Answer 1

In C++11, this in-class declaration:

static constexpr double cd = 3.0;

is not a definition (... until C++17; after which constexpr static data members are implicitly inline).

This is a out-of-class definition:

constexpr double Account::cd;

A definition is is needed if Account::cd is odr-used, which it is if it is passed to:

double GetCircumference(const double &dR);

as its reference is taken.

[basic.def.odr]/3 Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program; no diagnostic required. [...]

However, if it is passed to

double GetCircumference(const double dR);

it is not odr-used:

[basic.def.odr]/2 An expression is potentially evaluated unless it is an unevaluated operand (Clause [expr]) or a subexpression thereof. A variable whose name appears as a potentially-evaluated expression is odr-used unless it is an object that satisfies the requirements for appearing in a constant expression ([expr.const]) and the lvalue-to-rvalue conversion ([conv.lval]) is immediately applied.

and a definition is not required.