Difference between int x = 5; and int x = {5}

Question

Both of these statements work the same:

int x = 5;

int x = {5};

That's also true when I try:

char str[] = "hello";

char str[] = {"hello"};

How does the language define initializing a variable using curly braces?

Answer 1

There are three different cases of initialization which apply here:

• For plain variables/pointers, formally called scalars, then C17 6.7.9 §11 says that you can optionally add braces if you like:

  The initializer for a scalar shall be a single expression, optionally enclosed in braces.

• Then §14 mentions character arrays specifically - they may be initialized with a string literal. And again they may optionally have braces:

  An array of character type may be initialized by a character string literal or UTF–8 string literal, optionally enclosed in braces.

• Finally, §16 deals with all other initializers of aggregates (arrays or struct) and unions, braces are no longer optional but mandatory:

  Otherwise, the initializer for an object that has aggregate or union type shall be a brace-enclosed list of initializers for the elements or named members.

This makes it possible to write all initializer lists with consistent syntax and using {} no matter which type that the initializer list is initializing. Which in turn can perhaps be handy when writing type-generic macros and similar.