Why does the sizeof
operator return a size larger for a structure than the total sizes of the structure's members?
C++ Struct Size – Why sizeof Struct is Not Equal to Sum of sizeof Members
c++c++-faqsizeofstruct
c++c++-faqsizeofstruct
Why does the sizeof
operator return a size larger for a structure than the total sizes of the structure's members?
Best Answer
This is because of padding added to satisfy alignment constraints. Data structure alignment impacts both performance and correctness of programs:
SIGBUS
).Here's an example using typical settings for an x86 processor (all used 32 and 64 bit modes):
One can minimize the size of structures by sorting members by alignment (sorting by size suffices for that in basic types) (like structure
Z
in the example above).IMPORTANT NOTE: Both the C and C++ standards state that structure alignment is implementation-defined. Therefore each compiler may choose to align data differently, resulting in different and incompatible data layouts. For this reason, when dealing with libraries that will be used by different compilers, it is important to understand how the compilers align data. Some compilers have command-line settings and/or special
#pragma
statements to change the structure alignment settings.