CPL: C-style programming

C and C++ are closely related languages. They both originate in "Classic C" from 1978 and have evolved in ISO committees since then. Many attempts have been made to keep them compatible, but neither is a subset of the other.

C rule summary:

CPL.1: Prefer C++ to C

Reason

C++ provides better type checking and more notational support. It provides better support for high-level programming and often generates faster code.

Example
char ch = 7;
void* pv = &ch;
int* pi = pv;   // not C++
*pi = 999;      // overwrite sizeof(int) bytes near &ch

The rules for implicit casting to and from void* in C are subtle and unenforced. In particular, this example violates a rule against converting to a type with stricter alignment.

Enforcement

Use a C++ compiler.

CPL.2: If you must use C, use the common subset of C and C++, and compile the C code as C++

Reason

That subset can be compiled with both C and C++ compilers, and when compiled as C++ is better type checked than "pure C."

Example
int* p1 = malloc(10 * sizeof(int));                      // not C++
int* p2 = static_cast<int*>(malloc(10 * sizeof(int)));   // not C, C-style C++
int* p3 = new int[10];                                   // not C
int* p4 = (int*) malloc(10 * sizeof(int));               // both C and C++
Enforcement

  • Flag if using a build mode that compiles code as C.

    • The C++ compiler will enforce that the code is valid C++ unless you use C extension options.

CPL.3: If you must use C for interfaces, use C++ in the calling code using such interfaces

Reason

C++ is more expressive than C and offers better support for many types of programming.

Example

For example, to use a 3rd party C library or C systems interface, define the low-level interface in the common subset of C and C++ for better type checking. Whenever possible encapsulate the low-level interface in an interface that follows the C++ guidelines (for better abstraction, memory safety, and resource safety) and use that C++ interface in C++ code.

Example

You can call C from C++:

// in C:
double sqrt(double);
// in C++:
extern "C" double sqrt(double);
sqrt(2);
Example

You can call C++ from C:

// in C:
X call_f(struct Y*, int);
// in C++:
extern "C" X call_f(Y* p, int i)
{
    return p->f(i);   // possibly a virtual function call
}
Enforcement

None needed