C++ temporary class instantiation ambiguously

问题

Let we have procedure formed as class. Only constructor call makes some side effect. No need to handle class instance in memory after call. Following code instantiate that class:

struct A{
    A(int){}
    };
int main() {

    A(0);//right. Pass const to ctor
    int x=0;
    A(x);//bad. Compiler interpret like A x;
    (A(x));//right. New temporary object passed to brackets
    A((int)x);//right. Pass temporary int to ctor

    return 0;
}

(see also on Online IDE)

Why A(x); interpret as variable x declaration instead of temporary A object instantiaton?

回答1:

From the C++11 standard, ISO/EIC 14882 §6.8 [stmt.ambig] ¶1 (emphasis mine):

There is an ambiguity in the grammar involving expression-statements and declarations: An expression-statement with a function-style explicit type conversion (5.2.3) as its leftmost subexpression can be indistinguishable from a declaration where the first declarator starts with a (. In those cases the statement is a declaration.

To apply this to your question, A(x); can parse as either "call the function A / construct a temporary object of type A, and pass x as the only function/constructor argument" or "declare a variable x of type A." The standard says that in this case, it should be parsed as a variable declaration.

Your other examples are not ambiguous because they cannot be parsed as a variable declaration, and so they are parsed as a call to A's constructor.

回答2:

That's because what you consider should be the parameter list to the ctor, (x) is being interpreted as "x in parentheses". Thus, A(x) is read as A (x) is read as A x.

In the other cases, the compiler has a hint suggesting that it should generate an A instance, calling the ctor with the arguments supplied.

来源：https://stackoverflow.com/questions/29058769/c-temporary-class-instantiation-ambiguously