Cleaning up C code for C++
The next step in migration to C++ is to review and modify existing C code so that it is truly C++ compatible and can be processed by a C++ compiler without any problems. The resulting code may be called Clean C. Here are the key issues to consider:

1) Function prototypes " These are mandatory in C++.

2) Type casts " Implicit casting is much stricter in C++; use explicit cast operators like (char).

3) Enumerated types " These are not taken very "seriously" by C compilers, being considered a means of making typed, symbolic constants; in C++ enum declarations result in true data types.

4) String array size " In C and C++ a char array is commonly used to contain a NULL terminated character string. This requires the array to have sufficient capacity for the characters and the terminator.

In C, it is not permitted to initialize an array with a string larger than the declared array size, but the NULL terminator may be discarded. This leniency is not extended in C++. A simple strategy is to leave the array size unspecified and allow the compiler to allocate the space, thus:

            char str[] = "xyz";

5) Nested structures " In C, if a struct is defined within another struct, the inner struct may also be used declared by itself elsewhere in the module. In C++, the scope of the inner struct is strictly confined to the outer one. Hence, the following code would not be Clean C:

            struct out
            {
                        struct in { int i; } m;
                        int j;
            };
            struct in inner;
            struct out outer;

6) Multiple declarations " In C, it is quite legal to declare a variable in a module, outside of a function and then declare it again later in the module. This only causes problems if the redeclaration is inconsistent. In C++, multiple declarations are illegal.

7) Additional keywords " C++ introduces a number of additional keywords on top of those already reserved in C. This may be a problem if there is a clash with identifiers in the C code. The best strategy is to introduce a naming convention and apply it to the code. For example, all functions, types and variables have an upper case initial letter and all constants are fully capitalized.

C+: A better C?
The last step in the migration process, prior to considering object oriented techniques, is to incrementally apply C++ language features. This results in a hybrid language " a "better C" " that we might call C+. Possibilities include, but are not confined to, the following C++ features:

1) New comment notation " C++ introduces a new, end of line comment notation using the // operator. This is neater and addresses issues with nesting when using the traditional " /* ... */ " notation (which is still available, of course).

2) Reference parameters " In C++ by default, parameters are mostly passed by value (copy) the same as in C. However, there is also the option to pass by reference to avoid complications of explicit pointer usage.

3) Variable declaration placement " In C, automatic variables must be declared at the head of a block (commonly they are just placed at the head of a function). This limitation is relaxed in C++, enabling variables to be declared close to their point of first use. For example:

            for (int x=0; x<3; x++)
            ...

4) Constructors in structures " In C++, a class definition may include constructor and destructor member functions, which are automatically called when an object comes into and goes out of scope, respectively. Since a struct is, in most respects, identical to a class, it too may include a constructor. This is useful to facilitate the automatic initialization of the struct variable.

5) Memory leaks " By use of paired constructors and destructors, memory leaks may be avoided by ensuring that allocated resources (like dynamic memory) are deallocated when no longer required.

6) Exception handling " The EHS, introduced earlier in this paper, is not strictly an object oriented language feature and, hence, its use may logically be considered when using C+.

Next in Part 2: "Encpsulation of expertise using C++ objects."

Colin Walls has over twenty-five years experience in the electronics industry, largely involved with embedded software. A frequent presenter at conferences and seminars including the Embedded Systems Conference he is a member of the marketing team of the Embedded Systems Division of Mentor Graphics.

Click here to see Part 2.