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Standard-layout types can be used to communicate with code written in other programming languages, as the layout of the type is strictly specified. The C++ Standard, [class], paragraph 7 [ISO/IEC 14882-2014], defines standard-layout classes as ones that

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A special instance of this guidance involves non-C++ code compiled by a different compiler, such as C standard library implementations that are exposed via the C++ standard library. C standard library functions are exposed with C++ signatures, and the type system frequently assists in ensuring that types match appropriately. This process disallows passing a pointer to a C++ object to a function expecting a char * without additional work to suppress the type mismatch. However, some C standard library functions accept a void * for which any C++ pointer type will suffice. Passing a pointer to a nonstandard-layout type in this situation results in indeterminate behavior because it depends on the behavior of the other language as well as on the layout of the given object. For more information, see rule EXP64EXP56-CPP. Do not call a function with a mismatched language linkage.

Only pass a nonstandard-layout type object across execution boundaries when both sides of the execution boundary adhere to the same ABI. This is permissible if the same version of a compiler is used to compile both sides of the execution boundary, or if the compiler used to compile both sides of the execution boundary is ABI-compatible across multiple versions, or if the differing compilers document that they adhere to the same ABI.

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In this noncompliant code example, a pointer to an object of nonstandard-layout type is passed to a function that has a "Fortran" language linkage. Language linkages other than "C" and "C++" are conditionally supported with implementation-defined semantics [ISO/IEC 14882-2014]. If the implementation does not support this language linkage, the code is ill-formed. Assuming that the language linkage is supported, any operations performed on the object passed may result in indeterminate behavior, which could have security implications.

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Bibliography

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