c++ - Can initialization of static constant be skipped by 'case' label? - Stack Overflow

admin管理员组

文章数量:1345316

I have a constant declared in one case of my switch statement:

void foo( int& v ) {
    switch( v ) {
    case 0:
        static constexpr int c{ 0 };
        break; 
    case 1:
        v = c;
        break;
    }
}

Everything works fine in GCC, Clang and EDG. But if I compile the program in Visual Studio, it complains

error C2360: initialization of 'c' is skipped by 'case' label

Online demo:

Is it correct that initialization of the constant c can be skipped? Is the program really ill formed or it must be accepted?

I have a constant declared in one case of my switch statement:

void foo( int& v ) {
    switch( v ) {
    case 0:
        static constexpr int c{ 0 };
        break; 
    case 1:
        v = c;
        break;
    }
}

Everything works fine in GCC, Clang and EDG. But if I compile the program in Visual Studio, it complains

error C2360: initialization of 'c' is skipped by 'case' label

Online demo: https://gcc.godbolt./z/jTdnhfzoo

Is it correct that initialization of the constant c can be skipped? Is the program really ill formed or it must be accepted?

• c++
• switch-statement
• language-lawyer
• c++14
• constexpr

Share Improve this question Follow edited yesterday Fedor asked 2 days ago FedorFedor 21.6k3737 gold badges5555 silver badges168168 bronze badges 7

• 6 Looks ill-formed to me. – Eljay Commented 2 days ago
• This doesn't deal with constexpr variables so not sure if it applies: stackoverflow/questions/92396/… – NathanOliver Commented 2 days ago
• @NathanOliver The way I read the standard, constexpr doesn't matter. It's the constant initialization of a static in the block that makes it valid. It could be just static int c{0}; ...unless I'm missing something. – Ted Lyngmo Commented 2 days ago
• @TedLyngmo IDK. I'm still trying to find the text in the standard where it states that the code is ill form if control flow bypasses a block variable initialization. – NathanOliver Commented 2 days ago
• 3 Language-lawyer tag needed. – Red.Wave Commented yesterday

 |  Show 2 more comments

1 Answer 1

Sorted by: Reset to default 4

Can initialization of static constant be skipped by case label?

No, not if it's constant initialized as in your function.

C++14 6.7.4

Constant initialization ([basic.start.init]) of a block-scope entity with static storage duration, if applicable, is performed before its block is first entered.

This means that your function is valid and will initialize c when the switch block is first entered. A similarly valid function:

void foo(int& v) {
    goto bar;
    static constexpr int c{123};
bar:
    v = c; // assigns 123 to v
}

Removing constexpr doesn't matter here. It'll still be constant initialization of a block-scope entity with static storage duration and therefore performed when the block is first entered.

This bug in MSVC is reported here.

Note: Just because it's valid, it doesn't mean it's a good idea. Don't do this.

本文标签： cCan initialization of static constant be skipped by 39case39 labelStack Overflow