C++ Most vexing parse

Most vexing parse is the surprising rule that in contexts ambiguous between a declaration and a temporary, the compiler interprets it as a declaration. For example, Widget w(Widget()); declares w as a function returning Widget, not a Widget object initialized with a temporary.

Use brace initialization Widget w{}; to avoid ambiguity with function declarations.

This example shows the most vexing parse trap and how brace initialization avoids it.

// compile: g++ -o vexing vexing.cpp
// run: ./vexing
// description: most vexing parse: function declaration vs. temporary
 
#include <iostream>
 
class Widget {
public:
    Widget() { std::cout << "Widget constructed\n"; }
    void print() { std::cout << "Widget method\n"; }
};
 
int main() {
    // Most vexing parse: declares w as function returning Widget
    // Widget w(Widget());
 
    // These avoid the trap:
    Widget w1{};           // brace init: constructs w1
    Widget w2{Widget()};   // brace init with temp
    Widget w3 = Widget();  // assignment init
 
    w1.print();
 
    return 0;
}