c++ - 具有两个 int 列表的 structinit 减少为一个 int

Question

我试图用变量初始化一个结构。但是如果我使用 type name = (values...) 它只使用最后一个元素来初始化。它看起来不适合我，但我不知道它是否存在未定义的行为、编译器错误或其他问题。我会排除错误消息或用更多元素初始化。

代码:

struct funct {
    funct(int i)
    {
        std::cout << "init with one\t" << i << std::endl;
    }
    funct(int i, int j)
    {
        std::cout << "init with two\t" << i << "\t" << j << std::endl;
    }
};

int main() {
    funct tempa = funct(42);
    funct tempb = 43;
    funct tempc = funct(44, 45);
    funct tempd = (46, 47); // thats the compiling thing
    return 0;
}

输出:

init with one   42
init with one   43
init with two   44      45
init with one   47

为什么是这样而不是用 2 或编译器错误消息初始化？

编译器:g++ (GCC) 5.3.0

Answer 1

正如 Ben 所说，是逗号运算符导致了问题。

http://en.cppreference.com/w/cpp/language/operator_other#Built-in_comma_operator

In a comma expression E1, E2, the expression E1 is evaluated, its result is discarded, and its side effects are completed before evaluation of the expression E2 begins

通过执行 a = (b,c) 您可以有效地设置 a = c 并丢弃 b