Is `Timer()` a Constructor Call or a Function Call? The \'Most Vexing Parse\' Ambiguity in C

The "Most Vexing Parse" Ambiguity: Unraveling Uniform Initializers

In the realm of C , the "most vexing parse" ambiguity can arise when encountering uniform initializers, as exemplified by the following code:

<code class="cpp">class Timer {
public:
    Timer() {}
};

int main() {
    auto dv = Timer(); // What is Timer() ? And what type is dv?
    int time_keeper(Timer()); // This is a function right? And why isn't the argument "Timer (*) ()"?
    return 0;
}</code>

Interpreting dv

In the line auto dv = Timer();, the type of dv hinges on the interpretation of Timer(). This can either be:

• A constructor call, instantiating an object of type Timer.
• A function call to a hypothetical operator() overload in Timer, since uniform initialization can be used to invoke function calls.

In this case, it signifies a constructor call, resulting in dv being an object of type Timer. The auto keyword infers the type based on the initializer.

Understanding time_keeper

In the line int time_keeper(Timer());, a function named time_keeper is declared. The argument is not a function pointer of type Timer (*) (), as one might expect. Rather, it represents a pointer to a function that returns a Timer and takes no arguments. This is because:

• Functions implicitly decay to a pointer to function when passed as arguments.
• In this specific instance, the function call decays to a pointer to a function that returns a Timer and takes no arguments, i.e., Timer(*)().

In Conclusion

The "most vexing parse" ambiguity highlights the potential for confusion when interpreting certain constructs in C . Understanding the concept of uniform initialization and function decay is crucial for correctly comprehending such scenarios.

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