The All-in-One Guide to C++20/

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Default Member Initializers for Bit Fields

Default Member Initializers for Bit Fields

Learn how to default initialize the members of a bit field with an example.

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First of all, what is a bit field? Here is the definition from Wikipedia: “A bit field is a data structure used in computer programming. It consists of a number of adjacent computer memory locations which have been allocated to hold a sequence of bits, stored so that any single bit or group of bits within the set can be addressed. A bit field is most commonly used to represent integral types of known, fixed bit-width.”

With C++20, we can default initialize the members of a bit field:

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#include<iostream>
struct Class11 {
    int i = 1;
    int j = 2;
    int k = 3;
    int l = 4;
    int m = 5;
    int n = 6;
};
struct BitField20 {
    int i : 3 = 1;
    int j : 4 = 2;
    int k : 5 = 3;
    int l : 6 = 4;
    int m : 7 = 5;
    int n : 7 = 6;
};
int main() {
    std::cout << '\n';
    std:: cout << "sizeof(Class11): " << sizeof(Class11) << '\n';
    std:: cout << "sizeof(BitField20): " << sizeof(Class11) << '\n';
    std::cout << '\n';
}

According to the members of a class (lines 4 - 9) with C++11, the members of the bit field can have default initializers (lines 13 - 18) with C++20. When you sum up the numbers 33 ...