Compatibility of C and C++

Compatibility Of C And C++

The C and C++ programming languages are closely related. C++ grew out of C, as it was designed to be source-and-link compatible with C. Due to this, development tools for the two languages (such as IDEs and compilers) are often integrated into a single product, with the programmer able to specify C or C++ as their source language. However, due to minor semantic differences, most non-trivial C programs will not compile as C++ code without modification—C++ is not a strict superset of C.

Likewise, C++ introduces many features that are not available in C and in practice almost all code written in C++ is not conforming C code. This article, however, focuses on differences that cause conforming C code to be ill-formed C++ code, or to be conforming/well-formed in both languages but to behave differently in C and C++.

Bjarne Stroustrup, the creator of C++, has suggested that the incompatibilities between C and C++ should be reduced as much as possible in order to maximize inter-operability between the two languages. Others have argued that since C and C++ are two different languages, compatibility between them is useful but not vital; according to this camp, efforts to reduce incompatibility should not hinder attempts to improve each language in isolation. The official rationale for the 1999 C standard (C99) "endorse the principle of maintaining the largest common subset" between C and C++ "while maintaining a distinction between them and allowing them to evolve separately," and stated that the authors were "content to let C++ be the big and ambitious language."

Several additions of C99 are not supported in C++ or conflict with C++ features, such as variadic macros, compound literals, designated initializers, variable-length arrays, and native complex-number types. The long long int datatype and restrict qualifier defined in C99 are not included in the current C++ standard, but most mainstream compilers such as the GNU Compiler Collection, Microsoft Visual C++, and Intel C++ Compiler provide them as an extension. The long long datatype along with variadic templates, with which some functionality of variadic macros can be achieved, are present in the new C++ standard, C++11. On the other hand, C99 has reduced some other incompatibilities by incorporating C++ features such as // comments and mixed declarations and code.