Can constexpr and inline functions be re-defined?

Question

I am verifying a statement on C++ Primer that:

Unlinke other functions, inline and constexpr functions may be defined multiple times in the program.

I used two definitions of a constexpr cfunc() below, expecting foo_0() will call the 1st def while foo_1() will call the 2nd def. However the attempt failed with compilation error (in the end). WHy?

constexpr int cfunc(){
  return 42;
}

int foo_0(){
  return cfunc();
}

constexpr int cfunc(){
  return 42;
}

int foo_1(){
  return cfunc();
}


int main(int argc, char **argv) {

  cout << foo_0() << endl;  
  cout << foo_1() << endl;  

  /* testconstexprfunc2.cpp:24:15: error: redefinition of ‘constexpr int cfunc()’ */
  /* testconstexprfunc2.cpp:16:15: error: ‘constexpr int cfunc()’ previously defined here */

  return 0;

}

Answer 1

Yes unlike other functions, inline and constexpr functions may be defined multiple times in the program. However, the definitions must match exactly.

Justification according to the standard:

From § 7.1.5/2 The constexpr specifier [dcl.constexpr]:

constexpr functions and constexpr constructors are implicitly inline .

From § 3.2/6 One definition rule [basic.def.odr]:

There can be more than one definition of a class type (Clause 9), enumeration type (7.2), inline function with external linkage ... in a program provided that each definition appears in a different translation unit...

From § 7.1.2/4 Function specifiers [dcl.fct.spec]:

An inline function shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case.

Thus, since a constexpr function is implicitly inline it has all the attributes of an inline function. Therefore, constexpr functions can have more than one definition provided that each definition appears in a different translation unit .

Why your program fails:

In your case the program fails because you are violating this rule. That is, you redefine the same constexpr function in the same translation unit (ie, main.cpp).

Answer 2

An inline function can be defined multiple times in a program but only once in a translation unit. That means it can be defined only once in a header file. You can't have more than one definition in a translation unit even if they are identical.

Answer 3

It is true - regarding inline functions. This is the case to allow inline functions in a header, in order to make inlining possible at compile-time (instead at link time).

The following citation does not mention constexpr at all, but as noted they are implicitly inlined.

The key, as explained by @R Sahu, is provided that each definition appears in a different translation unit , which is not the case here.

Citing the standard, §3.2.6:

There can be more than one definition of (...) inline function with external linkage (7.1.2) (...) in a program provided that each definition appears in a different translation unit , and provided the definitions satisfy the following requirements. (...)