Can “T t = {};” and “T t{};” produce different results?

问题

The question is simple. Is it possible to construct such a type T, for which the following two variables declarations will produce different results?

T t1 = {};
T t2{};

I've been digging through the cppreference and the standard for more than an hour now, and I understood the following:

T t2{}; is a value initialization. No surprises here.
T t1 = {} is a list initialization with an empty braced-init-list.

But the last one is tricky since the "effects of list initialization" is an impressive... list. Which for classes, fundamental types and aggregates seems to boil down to value initialization. But I am not sure I haven't missed anything.

Maybe you can provide a context, in which the two declarations will have different effects?

UPD: Excellent answers about explicit constructors! Next level: is it possible that both statements compile, but have different effects on compile/run time?

回答1:

If you consider a case in which one statement will compile, but the other will not compile as "different effects," then yes, here's a context:

#include <iostream>

class T {
public:
    int data{ 0 };
    explicit T() {
        data = 0;
        std::cout << "Default constructor" << std::endl;
    }
};

int main()
{
    T t1 = {};
    T t2{};
    return 0;
}

The line declaring/initializing t1 gives the following, with clang-cl:

error : chosen constructor is explicit in copy-initialization

The MSVC compiler also complains:

error C2512: 'T': no appropriate default constructor available
message : Constructor for class 'T' is declared 'explicit'

回答2:

The difference is in explicit. I've managed to make msvc difference, but it looks like a compiler bug:

#include <iostream>
#include <initializer_list>

struct T
{
    template<class... A>
    T(A...) {std::cout << "1\n";}
    explicit T() { std::cout << "2\n"; }

};


int main()
{
    T t1 = {}; // 1
    T t2{}; // 2
}

来源：https://stackoverflow.com/questions/62008160/can-t-t-and-t-t-produce-different-results

标签

c++

initialization

language-lawyer