在变量声明中使用 for 循环

Question

Can I use for loop inside declaration a variable?

int main() {
    int a = {
        int b = 0;
        for (int i = 0; i < 5; i++) {
            b += i;
        }
        return b;
    };

    printf("%d", a);
}

Answer 1

You can use a lambda:

int main() {
    int a = []{
        int b = 0;
        for (int i = 0; i < 5; i++) {
            b += i;
        }
        return b;
    }();

    printf("%d", a);
}

It's important to note that you have to immediately execute it otherwise you attempt to store the lambda. Therefore the extra () at the end.

If you intent to reuse the lambda for multiple instantiations, you can store it separately like this:

int main() {
    auto doCalculation = []{
        int b = 0;
        for (int i = 0; i < 5; i++) {
            b += i;
        }
        return b;
    };

    int a = doCalculation();

    printf("%d", a);
}

If you need it in more than one scope, use a function instead.

Answer 2

actually has been prepared by C++ committee..
constexpr has many usefulness not yet exlpored

constexpr int b(int l) {
            int b=0;
            for (int i = 0; i < l; i++)
                b += i;
            return b;
        }

int main() {

    constexpr int a = b(5);

    printf("%d", a);
}