c++ - 赋值表达式是左值引用吗？

Question

例如:

template <typename T> void g(T &&val);
int i = 0; const int ci = i;
g(i = ci);

g 的模板参数是什么？

Answer 1

根据 §5.18/1:

The assignment operator (=) and the compound assignment operators all group right-to-left. All require a modifiable lvalue as their left operand and return an lvalue referring to the left operand. [...]

所以，在

g(i = ci)

左操作数 i 被返回，因此 T 被推导为 int&。

您可以通过以下代码段进行检查:

#include <type_traits>

template <typename T> 
void g(T &&val) {
    static_assert(std::is_same<T, int&>::value, "Nope");
}

int main() {
    int i = 0; const int ci = i;
    g(i = ci);
}

Live demo