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Sizeof
sizeof is a unary operator in the C and C++ programming languages that evaluates to the storage size of an expression or a data type, measured in units sized as char. Consequently, the expression sizeof(char) evaluates to 1. The number of bits of type char is specified by the preprocessor macro CHAR_BIT, defined in the standard include file <limits.h>. On most modern computing platforms this is eight bits. The result of sizeof is an unsigned integer that is usually typed as size_t.
The operator accepts a single operand which is either a data type expressed as a cast – the name of a data type enclosed in parentheses – or a non-type expression for which parentheses are not required.
Many programs must know the storage size of a particular datatype. Though for any given implementation of C or C++ the size of a particular datatype is constant, the sizes of even primitive types in C and C++ may be defined differently for different platforms of implementation. For example, runtime allocation of array space may use the following code, in which the sizeof operator is applied to the cast of the type int:
In this example, the malloc() function allocates memory and returns a pointer to the memory block. The size of the block allocated is equal to the number of bytes for a single object of type int multiplied by 10, providing space for ten integers.
It is generally not safe to assume the size of any datatype. For example, even though most implementations of C and C++ on 32-bit systems define type int to be four octets, this size may change when code is ported to a different system, breaking the code. The exception to this is the data type char, which always has the size 1 in any standards-compliant C implementation. In addition, it is frequently difficult to predict the sizes of compound datatypes such as a struct or union, due to padding. The use of sizeof enhances readability, since it avoids unnamed numeric constants (magic numbers).
An equivalent syntax for allocating the same array space results from using the dereferenced form of the pointer to the storage address, this time applying the operator to a pointer variable:
The operator sizeof produces the required memory storage space of its operand when the code is compiled. The operand is written following the keyword sizeof and may be the symbol of a storage space, e.g., a variable, an expression, or a type name. Parentheses for the operand are optional, except when specifying a type name. The result of the operator is the size of the operand in bytes, or the size of the memory storage requirement. For expressions, it evaluates to the representation size for the type that would result from evaluation of the expression, which is not performed.
For example, since sizeof(char) is defined to be 1 and assuming the integer type is four bytes long, the following code fragment prints 1,4:
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Sizeof
sizeof is a unary operator in the C and C++ programming languages that evaluates to the storage size of an expression or a data type, measured in units sized as char. Consequently, the expression sizeof(char) evaluates to 1. The number of bits of type char is specified by the preprocessor macro CHAR_BIT, defined in the standard include file <limits.h>. On most modern computing platforms this is eight bits. The result of sizeof is an unsigned integer that is usually typed as size_t.
The operator accepts a single operand which is either a data type expressed as a cast – the name of a data type enclosed in parentheses – or a non-type expression for which parentheses are not required.
Many programs must know the storage size of a particular datatype. Though for any given implementation of C or C++ the size of a particular datatype is constant, the sizes of even primitive types in C and C++ may be defined differently for different platforms of implementation. For example, runtime allocation of array space may use the following code, in which the sizeof operator is applied to the cast of the type int:
In this example, the malloc() function allocates memory and returns a pointer to the memory block. The size of the block allocated is equal to the number of bytes for a single object of type int multiplied by 10, providing space for ten integers.
It is generally not safe to assume the size of any datatype. For example, even though most implementations of C and C++ on 32-bit systems define type int to be four octets, this size may change when code is ported to a different system, breaking the code. The exception to this is the data type char, which always has the size 1 in any standards-compliant C implementation. In addition, it is frequently difficult to predict the sizes of compound datatypes such as a struct or union, due to padding. The use of sizeof enhances readability, since it avoids unnamed numeric constants (magic numbers).
An equivalent syntax for allocating the same array space results from using the dereferenced form of the pointer to the storage address, this time applying the operator to a pointer variable:
The operator sizeof produces the required memory storage space of its operand when the code is compiled. The operand is written following the keyword sizeof and may be the symbol of a storage space, e.g., a variable, an expression, or a type name. Parentheses for the operand are optional, except when specifying a type name. The result of the operator is the size of the operand in bytes, or the size of the memory storage requirement. For expressions, it evaluates to the representation size for the type that would result from evaluation of the expression, which is not performed.
For example, since sizeof(char) is defined to be 1 and assuming the integer type is four bytes long, the following code fragment prints 1,4: