How I can have variable number of parameters in my function in C++.
Analog in C#:
public void Foo(params int[] a) {
for (int i = 0; i < a.Length; i++)
Console.WriteLine(a[i]);
}
public void UseFoo() {
Foo();
Foo(1);
Foo(1, 2);
}
Analog in Java:
public void Foo(int... a) {
for (int i = 0; i < a.length; i++)
System.out.println(a[i]);
}
public void UseFoo() {
Foo();
Foo(1);
Foo(2);
}
These are called Variadic functions. Wikipedia lists example code for C++.
To portably implement variadic functions in the C programming language, the standard stdarg.h header file should be used. The older varargs.h header has been deprecated in favor of stdarg.h. In C++, the header file
cstdargshould be used.To create a variadic function, an ellipsis (
...) must be placed at the end of a parameter list. Inside the body of the function, a variable of typeva_listmust be defined. Then the macrosva_start(va_list, last fixed param),va_arg(va_list, cast type),va_end(va_list)can be used. For example:
#include <stdarg.h>
double average(int count, ...)
{
va_list ap;
int j;
double tot = 0;
va_start(ap, count); //Requires the last fixed parameter (to get the address)
for(j=0; j<count; j++)
tot+=va_arg(ap, double); //Requires the type to cast to. Increments ap to the next argument.
va_end(ap);
return tot/count;
}
The real C++ solution is variadic templates. You'll need a fairly recent compiler and enable C++11 support if needed.
Two ways to handle the "do the same thing with all function arguments" problem: recursively, and with an ugly (but very very Standards compliant) solution.
The recursive solution looks somewhat like this:
template<typename... ArgTypes>
void print(ArgTypes... args);
template<typename T, typename... ArgTypes>
void print(T t, ArgTypes... args)
{
std::cout << t;
print(args...);
}
template<> void print() {} // end recursion
It generates one symbol for each collection of arguments, and then one for each step into the recursion. This is suboptimal to say the least, so the awesome C++ people here at SO thought of a great trick abusing the side effect of a list initialization:
struct expand_type {
template<typename... T>
expand_type(T&&...) {}
};
template<typename... ArgTypes>
void print(ArgTypes... args)
{
expand_type{ 0, (std::cout << args, 0)... };
}
Code isn't generated for a million slightly different template instantiations, and as a bonus, you get preserved order of you function arguments. See the other answer for the nitty gritty details of this solution.