Thank you C++ for … initializer list

Most vexing parse

“Everything that resembles a function is a function.”

C++03

struct Bar {
};

struct Foo {
	Foo() : x(0) {}
	Foo(const Bar&) : x(1) {}
	Foo(const Bar&, const Bar&) : x(2) {}

	int x;
};

int main() {
	Foo a();
	Foo b(Bar());
	Foo c(Bar(), Bar());

	std::cout << a.x << std::endl;
	std::cout << b.x << std::endl;
	std::cout << c.x << std::endl;
}

Result

Line 17: request for member ‘x’ in ‘a’, which is of non-class type ‘Foo()’

Line 18: request for member ‘x’ in ‘b’, which is of non-class type ‘Foo(Bar (*)())’

Line 19: request for member ‘x’ in ‘c’, which is of non-class type ‘Foo(Bar (*)(), Bar (*)())’

C++11

int main() {
	Foo a{}; // or just: Foo a;
	Foo b{Bar()};
	Foo c{Bar(), Bar()};

	std::cout << a.x << std::endl;
	std::cout << b.x << std::endl;
	std::cout << c.x << std::endl;
}

Result

0

1

2

Container initialization

“It’s not easy to initialize a container, bar some fancy comma operator tricks.”

C++03

struct Foo {
	Foo() : _container() {
		_container.push_back(1);
		_container.push_back(2);
		_container.push_back(3);
		_container.push_back(4);
		_container.push_back(5);
		_container.push_back(6);
		_container.push_back(7);
	}

	// Alternative
	Foo(int) : _container(helper()) {
	}

	static std::vector<int> helper() {
		std::vector<int> result;
		result.push_back(1);
		result.push_back(2);
		result.push_back(3);
		result.push_back(4);
		result.push_back(5);
		result.push_back(6);
		result.push_back(7);
		return result;
	}

	std::vector<int> _container;
};

C++11

struct Foo {
	Foo() : _container{ 1, 2, 3, 4, 5, 6, 7 } {
	}

	std::vector<int> _container;
};

Brief constructors

You (almost) always have to specify the constructor name.

C++03

struct YouDontWantToSpellThis {
	YouDontWantToSpellThis(int a, int b) {
		;
	}
};

std::vector<YouDontWantToSpellThis> container;

container.push_back(YouDontWantToSpellThis(1, 2));

C++11

container.push_back({1, 2});

Tags: C++11x, initializer list