Ternary operator + C++11 constructor from initializer_list

问题

While developing an application, I had the following problem. I wanted to return an empty std::list<string> when a given function pointer was null, or the result of that function otherwise. This is a simplified version of my code:

typedef std::list<std::string> (*ParamGenerator)();

std::list<std::string> foo() {
    /* ... */
    ParamGenerator generator = ...;
    if(generator)
        return generator();
    else
        return {};
}

However, I usually like to use the ternary (?:) operator in these cases, so I tried using it this way (as usual):

return generator ? generator() : {};

But got this error:

somefile.cpp:143:46: error: expected primary-expression before ‘{’ token
somefile.cpp:143:46: error: expected ‘;’ before ‘{’ token

Does this mean I can't use the ternary operator to return objects created using their constructor from an initializer_list? Is there any particular reason for that?

回答1:

Standard writes in 8.5.4.1: List-initialization

Note: List-initialization can be used

as the initializer in a variable definition (8.5)

as the initializer in a new expression (5.3.4)

in a return statement (6.6.3)

as a function argument (5.2.2)

as a subscript (5.2.1)

as an argument to a constructor invocation (8.5, 5.2.3)

as an initializer for a non-static data member (9.2)

in a mem-initializer (12.6.2)

on the right-hand side of an assignment (5.17)

Nothing of them is a ternary operator. The more minimalistic return 1?{}:{}; is invalid too, what you want is impossible.

Of course you can explicitly call the constructor std::list<std::string>{}, but I would recommend to write out the if-else-block as you already did.

回答2:

When you do {} the compiler has no knowledge of the type you are expecting, so it's just a meaningless expression that the compiler doesn't know what to do with. Both sides of the : are evaluated separately, and only then will the compiler complain if the types don't match. I would just do this:

return generator ? generator() : std::list<std::string>();

回答3:

If you really like the ternary operator, you can try something like this:

return generator ? generator() : decltype(generator()) { "default value", "generator was empry" };

it will work even if you change the return types later.

回答4:

Another possibility is to define a wrapper function for the conditional operator:

template<class T> T& conditional(bool b, T&x, T&y) { return b ? x : y; }
template<class T> const T& conditional(bool b, const T&x, const T&y) { return b ? x : y; }

That allows you to call:

return conditional(generator, generator(), {});

来源：https://stackoverflow.com/questions/9982681/ternary-operator-c11-constructor-from-initializer-list

标签

c++

constructor

c++11

ternary-operator

initializer-list