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    <p>While the struct can be padded, on all systems I know, the compiler will pad so that the alignment of the structure is the same as the largest alignment of its members. It does this so that an array of the structure will always be correctly aligned. </p> <p>So:</p> <pre><code>struct S { char a; } // Size 1, no padding struct S2 { unsigned int a; char b; } // Size 8, 3 bytes padding (assuming 32 bit integer) </code></pre> <p>Edit: Note, that compilers can also add internal padding, to keep the alignment of the data correct.</p> <p>The C/C++ standard doesn't specify any of these detail. What you want is the C ABI (application binary interface) for the system you're running on, which should specify default layout for structs (compilers can choose to override this if they see fit, see also #pragma pack). For an example, look at the <a href="http://x86-64.org/documentation/abi.pdf" rel="nofollow">X86_64 ABI</a> page 13, which states:</p> <blockquote> <p>Aggregates and Unions Structures and unions assume the alignment of their most strictly aligned compo- nent. Each member is assigned to the lowest available offset with the appropriate alignment. The size of any object is always a multiple of the object‘s alignment. An array uses the same alignment as its elements, except that a local or global array variable of length at least 16 bytes or a C99 variable-length array variable always has alignment of at least 16 bytes. Structure and union objects can require padding to meet size and alignment constraints. The contents of any padding is undefined.</p> </blockquote>
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    1. POCan wrapping a type in a struct cause additional padding?
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    1. USDave S
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    1. POCan wrapping a type in a struct cause additional padding?
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    1. COThis question is not about structs with multiple data members.
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      1. USfredoverflow
    2. COIndeed. I thought also that the fields *may* appear in memory in any order; the compiler is free to order the fields in whatever order it likes and ignore the order you declared them in. I wouldn't expect it to be very common.
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      1. USNicholas Wilson
    3. CO@Nicholas: data members of POD structs are guaranteed to appear in memory in the order they're declared. IIRC, for non-POD the rule is that in between two access specifiers, data members appear in order, although that might just be public members.
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      1. USSteve Jessop
 

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