How Does C Determine the Memory Layout of Objects?

Memory Layout of C Objects

In C , the layout of objects in memory is not explicitly defined by the language standard, but instead left largely to the implementation. However, there are some key principles that guide the way in which objects are stored.

One important factor is the order of member variables. According to the C standard (§ 9.2.14), non-static member variables of a class with the same access specifier will be allocated in the order of their declaration. This means that the first member variable declared will be located at the lowest memory address, followed by the second member variable, and so on.

Another aspect of memory layout involves the allocation of space for class members, subobjects of base classes, virtual function management, and padding and alignment of these data. While the implementation has some flexibility in this regard, a widely used specification is the Itanium ABI, which is adhered to by compilers such as gcc and clang.

The Itanium ABI details the memory layout of classes and structs, including the placement of member variables, virtual function tables, and other data. However, it's important to note that the Itanium ABI is not part of the C standard and may not be applicable to all implementations.

To obtain more specific information about the memory layout of a particular class or struct, one can consult the documentation or use tools provided by the compiler. For example, clang offers a tool that allows you to view the memory layout of classes.

By understanding the principles governing memory layout, developers can better appreciate the consequences of accessing class members and performing operations involving pointers and virtual functions.

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