With this definition

struct A {
    int x;
};

why is

struct A a = {0};

valid syntax while

struct A a;

a = {0};

produces the error, "expected expression before '{' token"?

If I replace that line with

a = (struct A){0};

then everything is fine so it's not a problem with assigning to structures.

CodePudding user response：

The curly brace syntax {} designates an initializer list, and such a list can only be used in an initialization.

In this case:

struct A a = {0};

a is being declared and initialized with {0} and is valid syntax.

This is not valid:

a = {0};

Because this is an expression an initializer list can't appear in an expression.

This:

a = (struct A){0};

Is not a cast but an example of a compound literal. The syntax is similar to that of a cast, but what it is doing instead is creating an object of type struct A and giving an initializer for it. This object is then assigned to a.

CodePudding user response：

You can only use initializers in a declaration, so statement a = {0}; is invalid.

In statement a = (struct A){0}; you are actually assigning using a compound literal expression, which is valid.