G++ (C++14) linker error on working C++03 code

Question

Consider the following piece of code.

class aClass
{
public:
    static const int HALLO = -3;
};

int main()
{
  std::vector<double > a;
  std::vector<int> b;
  std::vector<int> c;
  int d = aClass::HALLO; //fine
  a.resize(10,aClass::HALLO); //fine
  b.resize(10,aClass::HALLO); // linker error c++11 and c++14
  c.resize(10,(int)(double)aClass::HALLO); //fine
  std::cout<<a[0]<<endl;
  std::cout<<b[0]<<endl;
  std::cout<<c[0]<<endl;
  return 0;
}

Compiling works with C++03 and yields the output:

-3
-3
-3

Compiling with C++11 or C++14, however, leads to a linker error:

/tmp/cc3BARzY.o: In Funktion `main':
main.cpp:(.text+0x66): Nicht definierter Verweis auf `aClass::HALLO'
collect2: error: ld returned 1 exit status

Weirdly enough, this only happens for vector b. If there is a cast to double(a) or even to double and back to int (c), the code runs as expected.

How can this behaviour be explained?

Answer 1

Until C++11 the signature of std::vector::resize() was

void resize( size_type count, T value = T() );

Now it is instead

void resize( size_type count, T const& value );

The change from pass-by-value to pass-by-const-ref causes the callsite to ODR-use aClass::HALLO where previously it did not. Casting to double then back to int yields a temporary in a way that avoids ODR-use; the call to a.resize() works for the same reason, as the int value is implicitly cast to a double and the argument reference is bound to the resulting temporary.

The usual fix here is to provide a definition for aClass::HALLO; if for some reason that's undesirable for you, a shorthand for yielding a temporary to avoid ODR-use is to apply unary operator+:

b.resize(10, +aClass::HALLO);