问题描述
你好，
在 C++ 中是否有一些简单的方法来判断(在编译时)一个类/结构是否没有数据成员?
is there some easy way in C++ to tell (in compile-time) if a class/struct has no data members?
例如struct T{};
我的第一个想法是比较 sizeof(T)==0，但这似乎总是至少为 1.
My first thought was to compare sizeof(T)==0, but this always seems to be at least 1.
显而易见的答案是只看代码，但我想打开它.
The obvious answer would be to just look at the code, but I would like to switch on this.
推荐答案
从 C++11 开始，您可以使用标准的 std::is_empty trait:https://en.cppreference.com/w/cpp/types/is_empty
Since C++11, you can use standard std::is_empty trait: https://en.cppreference.com/w/cpp/types/is_empty
如果您正在使用旧式编译器，有一个技巧:您可以从另一个空的类中派生并检查是否 sizeof(OtherClass) == 1.Boost 在它的 is_empty 类型特征中做到了这一点.
If you are on paleo-compiler diet, there is a trick: you can derive from this class in another empty and check whether sizeof(OtherClass) == 1. Boost does this in its is_empty type trait.
未经测试:
template <typename T>
struct is_empty {
struct helper_ : T { int x; };
static bool const VALUE = sizeof(helper_) == sizeof(int);
};
然而，这依赖于空基类优化(但所有现代编译器都这样做).
However, this relies on the empty base class optimization (but all modern compilers do this).
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