How Are C Struct Members Initialized?

Initialization of C Struct Members

C structs are user-defined types that group data of various types together. A common question arises regarding the default initialization of struct members. Do they automatically default to zero or require explicit initialization?

Default Initialization

In C , members of a struct are not initialized to zero by default. This means that uninitialized struct members can contain garbage values. To avoid this, you need to explicitly initialize the members to the desired values.

Initialization Options

There are several ways to initialize struct members:

• Aggregate Initialization: This is the most straightforward method and involves using curly braces to enclose the member values directly in the struct declaration.

struct Snapshot {
    double x; 
    int y;
};

Snapshot s = {0,0}; // Initializes x=0 and y=0

• Value Initialization: This method uses a special initializer list with empty braces {} to initialize all members to their default values. In the case of numeric types like double and int, this results in a value of zero.

struct Snapshot {
    double x; 
    int y;
};

Snapshot s = {}; // Initializes x=0 and y=0

• Constructor Initialization: If the struct has a user-declared constructor, you can initialize the members within that constructor.

struct Snapshot {
    int x;
    double y;
    Snapshot():x(0),y(0) { }
};

Snapshot s; // Initializes x=0 and y=0

Recursive Initialization:

Value initialization is recursive, meaning that it also initializes members of nested structs. For instance:

struct Parent {
    Snapshot s;
};

Parent p = {}; // Initializes p.s.x=0 and p.s.y=0

Conclusion:

Members of a C struct are not initialized to zero by default. You must explicitly initialize them using one of the methods described above to avoid undefined behavior.

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