"Inline" static declaration of object with initializer list

Question

Sorry for the awkward title, but I couldn't find a better one.

Consider this sample code (it has no purpose except illustrating the question):

#include <vector>

void FooBar(int);

void func1()
{
    static std::vector<int> vec {1, 2, 3, 4};

    for (auto & v : vec)
      FooBar(v);
}

void func2()
{
    for (auto & v : std::vector<int> {1, 2, 3, 4})
      FooBar(v);
}

The disassembly of this can be found here

In func1 the static vec vector is supposed to be constructed once and for all at startup. Actually the disassembly on godbolt mentioned above shows that the initialisation of the static vec is done only upon the first call of func1 and not at startup, but that's not the point here.

Now consider func2: here the vector is directly declared "inline" (not sure how this is actually called) inside the for statement, but of course that vector is constructed every time func2 is called.

Is there a way to declare that vector statically and inside the for statement, like for (auto & v : static std::vector<int> { 1, 2, 3, 4}) which is unfortunately not legal C++.

Answer 1

This won't actually help you yet. But there's something you can do in C++2a. It is, however, based on something that exists already in C++14 and C++17.

C++2a added an init-statement to the range based for loop. That's the new bit, the old bit is that it's the same init-statement as it is defined today. And the definition is as follow ([stmt.stmt]):

init-statement:
    expression-statement
    simple-declaration

What I'm going for is that a simple-declaration may contain a static qualifier. Yup, it does what you'd expect. So in C++2a you may write:

for (static std::vector<int> vec {1, 2, 3, 4}; int v : vec) {
   // Do things.
}

And if you want to test compiler support for it today, here's a C++17 kludge:

if (static std::vector<int> vec {1, 2, 3, 4}; true) {
    // init-statement in if was added in C++17
  for(int v : vec)
    FooBar(v);
}

And its disassembly.