The Most Vexing Parse: Timer() - Object or Function Call?
Oct 31, 2024 pm 06:09 PM
Most Vexing Parse: Unraveling Ambiguity in C 11
The "most vexing parse" ambiguity in C 11 presents itself when using uniform initializers, as evidenced in the following code snippet:
<code class="cpp">#include <iostream>
class Timer
{
public:
Timer() {}
};
int main()
{
auto dv = Timer(); // Ambiguity: Object or function call?
int time_keeper(Timer()); // Ambiguity: Pointer or call?
return 0;
}</code>
Understanding the First Expression (auto dv = Timer())
In the first expression, the auto keyword implies that the type of dv is inferred from the initializer on the right side of the equal sign (=). The initializer is a call to the Timer constructor with no arguments, which returns a Timer object. Therefore, dv is an object of type Timer.
Understanding the Second Expression (int time_keeper(Timer()))
In the second expression, the ambiguity arises because the compiler cannot determine whether Timer() is a function call or an object of type Timer passed by reference.
- If Timer() is a function call, then int time_keeper(Timer()) declares a function called time_keeper that takes a Timer object as input and returns an int.
- If Timer() is an object of type Timer, then int time_keeper(Timer()) declares a function called time_keeper that takes a pointer to a Timer object as input and returns an int.
However, because functions decay to pointers when passed as arguments, the true type of time_keeper is int(Timer(*)()), which resolves the ambiguity in favor of the pointer-to-function interpretation.
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