An initializer is C syntax that augments a declaration with an initial value for an object. A declaration names the kind of object an identifier refers to; a definition provides storage, and an object definition with an initializer does both. For scalar types, the initializer is an expression convertible to the object’s type, optionally enclosed in braces. section 2.3 section 5.5
Scalar and designated array initialization
double a = 7.8; double b = 2 * a; double A[5] = { [0] = 9.0, [1] = 2.9, [4] = 3.E+25, [3] = .00007, }; The designated entries select array indices explicitly. The missing element A[2] is initialized to 0.0, and indexing begins at zero. section 2.3 section 5.5
Use initializer forms according to the object being defined:
- Initialize scalar objects with a convertible expression, such as 7.8 or 2 * a. section 5.5
- Initialize arrays and other aggregate data with brace-enclosed lists; prefer designated initializers because explicit positions remain robust when declarations change. section 5.5
- Use {} as the default initializer for any object in C23; before C23, {0} was required for this general form. section 5.5
- Remember that static- and thread-storage objects receive default initialization, while automatic and allocated objects require explicit initialization. section 13.4
double A[5] = {
[0] = 9.0,
[1] = 2.9,
[4] = 3.E+25,
[3] = .00007,
};
Inspect a complete designated initializer for a five-element array, including explicit indices, omitted elements, and floating-point values. section 2.3
