核心常量表达式和数组索引

Question

参考以下代码段：

int main() {
    const std::size_t tabsize = 50;
    int tab[tabsize]; // OK: tabsize is a constant expression

    std::size_t n = 50;
    const std::size_t sz = n;
    int tab2[sz]; // error: sz is not a constant expression
                  // because sz is not initialized with a constant expression
}

当我使用 (gcc 4.2) 编译上面的代码段时：

g++ -ggdb -pedantic -std=c++14 -Wall

对于代码中突出显示为错误的行，仅生成以下警告：

 warning: variable length arrays are a C99 feature
      [-Wvla-extension]
    int tab2[sz]; // error: sz is not a constant expression
            ^

这似乎与上述 link 中概述的要求相矛盾，即核心常量表达式不得计算左值->右值隐式转换，除非值：

has integral or enumeration type and refers to a complete non-volatile const object, which is initialized with a constant expression

感谢您的想法。

Answer 1

支持可变长度数组是一种编译器扩展，标准通常允许这种扩展，只要它不会使一致的代码表现不同，并且只要它们说它是一个扩展（GCC 对该警告进行了处理）：

A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any well-formed program. Implementations are required to diagnose programs that use such extensions that are ill-formed according to this document. Having done so, however, they can compile and execute such programs.

^{[intro.compliance]/8}

如果需要，您可以通过 -pedantic-errors 禁用所有扩展。

核心常量表达式和数组索引

Core constant expression and array indexing

c++

constant-expression