Compatibility of C and C++

The <a href="/facts/C_/Ky2No763">C</a> and <a href="/facts/C%252B%252B/Et0F2qn9">C++</a> <a href="/facts/Programming_language/cWTYbgWa">programming languages</a> are closely related but have many significant differences. C++ began as a <a href="/facts/Fork_(software_development)/D6khxhpr">fork</a> of an early, pre-<a href="/facts/ANSI_C/rPKeEuPm">standardized</a> C, and was designed to be mostly source-and-link compatible with C compilers of the time. Due to this, development tools for the two languages (such as <a href="/facts/Integrated_development_environment/jbEINSQf">IDEs</a> and <a href="/facts/Compiler/WNfCDFJe">compilers</a>) are often integrated into a single product, with the programmer able to specify C or C++ as their source language. 
However, C is not a <a href="/facts/Subset/SJGERmbA">subset</a> of C++, and nontrivial C programs will not compile as C++ code without modification. 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++.
<a href="/facts/Bjarne_Stroustrup/Be1Yq6rn">Bjarne Stroustrup</a>, the creator of C++, has suggested that the incompatibilities between C and C++ should be reduced as much as possible in order to maximize interoperability 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 (<a href="/facts/C99/o5uMWjTQ">C99</a>) "endorse[d] 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 the current C++ standard or conflicted with C++ features, such as <a href="/facts/Variable-length_array/eYknlUTc">variable-length arrays</a>, native <a href="/facts/Complex_data_type/K9cXB0KR">complex number</a> types and the <a href="/facts/Restrict/WjVKK08w">restrict</a> <a href="/facts/Type_qualifier/7tUPjsua">type qualifier</a>. On the other hand, C99 reduced some other incompatibilities compared with C89 by incorporating C++ features such as // comments and mixed declarations and code.

Compatibility of C and C++ open-in-new

Compatibility of C and C++