10. Declarations
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Declarations and definitions
Declaration - tells the compiler what should it think about X.
- For variables: tells the compiler that the variable with a certain type exists (defined somewhere).
- For functions: tells the signature (return type and parameter list). The compiler knows whether conversion is required on parameter passing.
- For types: tells the compiler, that a type (struct, enum, etc. exists).
Definition - tells the compiler what is X in details.
- For variables: tells the type, the life and the scope. Allocate memory for that object.
- For functions: describe parameter list, return type and the algorithm to execute.
- For types: describe the data structure, and members.
The generic form of definitin/declaration is: storage-class type-name declarator-list
where declarator-list is: declarator , declarator …
storage-class is: auto, register, static, extern, typedef
Do not use auto and register. Auto has a different meaning in C++11 and register optimizations are done by modern compilers automatically.
The form of declarator is:
- function: decl ( param-list )
- pointer: * decl
- reference &decl
- array: decl [ n ]
- otherwise: id
Definition
Every C++ element should be defined exactly once, this rule is called as the One definition rule (ODR).
Functions should be defined outside of other functions; i.e. there are no inner functions. Variables can be defined outside (global variables) or inside (local variables) of functions.
In C++ the place of a variable definition can be anywhere, even between two executable statements:
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void func()
{
printf("Hello");
int i = 0; // definition and initialization
while ( i < 10 )
{
//...
++i;
}
}
Variable and function definitions
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int i; // integer variable
int *pi; // pointer to integer
int &ri = i; // reference to integer, must be initialized
int t[10]; // array of 10 integers
int func(){...} // function without parameters, returning int
int func(void){...} // also means no parameter, C reverse compatible
int func(int i, double d){...} // function, i and d are formal parameters
void func(){...} // function without parameters, no return type
//...
char *getenv(const char *var){...}
These definitions can be used recursively
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int **ppi; // pointer to a pointer to int
int tt[10][20]; // array of 10 arrays of 20 integers (200 int elements)
int *func(int){...} // int parameter function returning pointer to int
In case of disambiguity, usual precedence rules decide the meaning. We can use parenthesis to overrule precedence.
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int *ptr_arr[10]; // array of 10 pointers to integers
int (*ptr_to)[10]; // pointer to an array of 10 integers
But there are a few exeptions:
- No array of functions (but there is array of function pointers).
- No functions returning functions (but can return with function pointer).
Pointers to functions should define full signature.
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double (*funcptr)(double); // pointer to double f(double) func, like sin
//...
extern double sin(double); // declaration of sin, usually in <math.h>
funcptr = sin; // name of function is pointer to function value
Sometimes typedef is used to simplify definitions/declarations.
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typedef double length_t; // length_t is synonym of double
length_t f(length_t){...} // f has a double parameter and returns double
//...
typedef double (*trigfp_t)(double);// trigfp_t is pointer to function
// signature is part of the type
trigfp_t inverse(trigfp_t){...} // function returning pointer to function
int trigfp_t[10]; // array of 10 pointer to double(double) function
Arrays with automatic life (local, non-static arrays) can be variadic, i.e. the size of an array can be a value of a variable.
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static int t1[10]; // dimension must be known compile time
void f()
{
static int t2[10]; // dimension must be known compile time
int n;
// read n
int t3[n]; // ok
}
Declaration
Every C++ element should be declared before the first usage. Multiple declarations are allowed.
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extern int i; // integer variable is defined somewhere else
extern int *pi; // pointer to integer is defined somewhere else
extern int t[10]; // array of 10 integers
extern int t[]; // array of ? integers
extern int tt[10][20]; // array of 10x20 integers
extern int tt[][20]; // ?x20 integers, only leftmost dim can be omitted
extern int func(); // function without parameters, returning int
int func(); // same, compiler understands: this is declaration
int func(void); // also means no parameter, C reverse compatible
static int func(); // func has no external linkage, parameters unknown
//...
/* extern */ char *getenv();
Initialization
Variables can be initialized when defined. Actually, the best strategy is to delay definitions until we use and initialize the variable.
References and constant variables should be initialized!
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int i = 1;
int &ir = i; // reference must be initialized
const double pi = 3.14; // const must be initialized
extern double sin(double); // declaration
double (*funcptr)(double) = sin;
int arr1[10] = {0,1,2,3,4,5,6,7,8,9}; // ok, but hard to maintain
int arr2[10] = {0,1,2,3,4,5,6,7,8}; // arr2[9] == 0
int arr3[] = {0,1,2,3,4,5,6,7,8}; // int arr3[9]
char str1[] = {'H','e','l','l','o','\0'}; // char str1[6]
char str2[] = "Hello"; // char str2[6]
Forward declarations
There are cases, when two type definitions should refer to each other, therefore we can not serialize their declarations. In these situations (and some other cases) we shoudl use forward declarations.
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struct Current;
struct Other
{
Current *cp;
};
struct Current
{
Other *op;
};
External contants
Constants are local to the translation unit by default. However, we can define and declare cross-translation unit constants.
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const int bufsize = 1024; // non-extern const
extern const int global_const; // extern const declaration
extren const int global_const = 80; // extern const definition
Extern constants should be defined in a single translation unit, here will be the memory allocated for the constant. This is the place when we initialize it (line 2). In the other translation units we should declare the constant (line 3).
Pitfalls
There are different styles to declare pointers regarding where we put the start declarator. However, sometimes the syntax can be misleading.
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int i, j; // i and j are integers
int* ip, jp; // ip is pointer, but jp is integer
int *ip, *jp; // ok, both ip and jp are pointers
In the second case, only ip will be declared to pointer, the jp variable will be integer.