DevTut

# Compound Literals

# Definition/Initialisation of Compound Literals

A compound literal is an unnamed object which is created in the scope where is defined. The concept was first introduced in C99 standard. An example for compound literal is

# Examples from C standard, C11-§6.5.2.5/9:

int *p = (int [2]){ 2, 4 };

p is initialized to the address of the first element of an unnamed array of two ints.

The compound literal is an lvalue. The storage duration of the unnamed object is either static (if the literal appears at file scope) or automatic (if the literal appears at block scope), and in the latter case the object's lifetime ends when control leaves the enclosing block.

void f(void)
{
    int *p;
    /*...*/
    p = (int [2]){ *p };
    /*...*/
}

p is assigned the address of the first element of an array of two ints, the first having the value previously pointed to by p and the second, zero.[...]

Here p remains valid until the end of the block.

# Compound literal with designators

(also from C11)


 struct point {
    unsigned x;
    unsigned y;
  };

  extern void drawline(struct point, struct point);

 // used somewhere like this
 drawline((struct point){.x=1, .y=1}, (struct point){.x=3, .y=4});

A fictive function drawline receives two arguments of type struct point. The first has coordinate values x == 1 and y == 1, whereas the second has x == 3 and y == 4

# Compound literal without specifying array length

int *p = (int []){ 1, 2, 3};

In this case the size of the array is no specified then it will be determined by the length of the initializer.

# Compound literal having length of initializer less than array size specified

int *p = (int [10]){1, 2, 3};

rest of the elements of compound literal will be initialized to 0 implicitly.

# Read-only compound literal

Note that a compound literal is an lvalue and therefore it's elements can be modifiable. A read-only compound literal can be specified using const qualifier as (const int[]){1,2}.

# Compound literal containing arbitrary expressions

Inside a function, a compound literal, as for any initialization since C99, can have arbitrary expressions.

void foo()
{
    int *p;
    int i = 2; j = 5;
    /*...*/
    p = (int [2]){ i+j, i*j };
    /*...*/
}

# Syntax

  • (type){ initializer-list }

# Remarks

C standard says in C11-§6.5.2.5/3:

A postfix expression that consists of a parenthesized type name followed by a brace enclosed list of initializers is a compound literal. It provides an unnamed object whose value is given by the initializer list.99)

and footnote 99 says:

Note that this differs from a cast expression. For example, a cast specifies a conversion to scalar types or void only, and the result of a cast expression is not an lvalue.

Note that:

String literals, and compound literals with const-qualified types, need not designate distinct objects.101)

101) This allows implementations to share storage for string literals and constant compound literals with the same or overlapping representations.

Example is given in standard:
C11-§6.5.2.5/13:

Like string literals, const-qualified compound literals can be placed into read-only memory and can even be shared. For example,

(const char []){"abc"} == "abc"

might yield 1 if the literals’ storage is shared.

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