Initializing a static map using static members

Question

Trying to initialize a static map. From other questions I've gathered that it must be done outside of the header file, and in c++11 can be done with uniform initialization. But when I try to use another static member, I get problems:

foo.h:

#include <map>

class TestSuite {
    static constexpr int x = 3;
    static std::map<int, int> v; 
};

foo.cpp:

#include "foo.h"

std::map<int, int> TestSuite::v = {{x, 5}};

int main() {
    TestSuite t;
}

Then I get the error

In function `__static_initialization_and_destruction_0(int, int)':
foo.cpp:(.text+0x4b): undefined reference to `TestSuite::x'
collect2: error: ld returned 1 exit status

seems to work fine, what compiler do you use? – IceFire Jun 13 '16 at 14:46 — IceFire, Jun 13 '16 at 14:46
5.2, must be a new feature in 6.1 – acornellier Jun 13 '16 at 17:24 — acornellier, Jun 13 '16 at 17:24

score 4 · Accepted Answer · answered Jun 13 '16 at 15:19

I cannot reproduce this problem with GCC 6.1.0. However, it can be reproduced any time you try to bind a reference to a constexpr variable you haven't defined, which is probably what your std::map constructor does :

struct Foo {
    static constexpr int i = 42;
};

int main() {
    auto const &p = Foo::i; // undefined reference to `Foo::i'
}

This is because binding the reference is an ODR-use of i, which requires a unique definition to exist at link-time.

There is a simple workaround that works in most such cases, and that is to apply a unary +:

struct Foo {
    static constexpr int i = 42;
};

int main() {
    auto const &p = +Foo::i; // OK!
}

Applying + does not ODR-use i, since only its value is needed, not its identity. Then the reference binds to the temporary returned by +, instead of i.

score 3 · Answer 2 · answered Jun 13 '16 at 13:45

3

You also need to define TestSuite::x. Put this in your cpp file:

const int TestSuite::x;

answered Jun 13 '16 at 13:45

Kerrek SB

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Does static initialization order fiasco come into play at all? – NathanOliver Jun 13 '16 at 13:49
2

@NathanOliver if both are in the same TU, then order of initialization is well-defined. – Revolver_Ocelot Jun 13 '16 at 13:59

score 1 · Answer 3 · answered Jun 13 '16 at 14:38

1

Another solution.

Change the static member variable to a static member function.

class TestSuite {
    static constexpr int getX() { return 3;}
    static std::map<int, int> v; 
};

and

std::map<int, int> TestSuite::v = {{getX(), 5}};

answered Jun 13 '16 at 14:38

R Sahu

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Initializing a static map using static members

3 Answers3