(C++) Function

A function is "a named sequence of statements that can be invoked/called given arguments and that might return a value. The type of the function includes the number and types of argument and the type of the value returned, if any" [1].

[return type] [function name]( [arguments] )
{
  [statements]
}

A function has a declaration and a definition. The compiler needs to have read the function declaration before this function is used. If the definition is nowhere provided, this results in a linking error.

include <iostream>

include <vector>


//Declaration of calculateSum
double calculateSum(const std::vector<double>&); 
int main()
{
  std::vector<double> myVector;
  myVector.push_back(1.1);
  myVector.push_back(2.2);
  myVector.push_back(3.3);
  myVector.push_back(4.4);
  //Compiler trusts that calculateSume exists somewhere...
  std::cout << "myVector has sum: " << calculateSum(myVector) << std::endl;
}
//Definition of the function
//Removing this part returns in a linking error
double calculateSum(const std::vector<double>& anyVector)
{
  double sum = 0.0;
  const int size = anyVector.size();
  for (int i=0; i<size; ++i)
  {
    sum+=anyVector[i];
  }
  return sum;
}

Functions can also be static. This means that it can only be called from its unit/module.

static void sayHello()
{
  std::cout << "Hello" << std::endl;
}

Reading and using functions

The keyword const can be used to specify in the declaration what the function does.

For example, consider a C-style function that takes three pointers as an argument:

void SomeFunction(MyStruct1 * p1, MyStruct2 * p2, MyStruct3 * p3);

A const-correct programmer immediatly assumes that all three structures will be modified.

Looking further in the code, however:

void SomeFunction(MyStruct1 * p1, MyStruct2 * p2, MyStruct3 * p3)
{
  //In : p1 and p2, will not be changed
  //Out: p3
}

Using a comment does not enforce the compiler to give a compile error when the 'unchanged' structures ARE changed.

Using const will make the compiler complain. Also, it makes the comments obsolete:

void SomeFunction(const MyStruct1 * p1, const MyStruct2 * p2, MyStruct3 * p3);

Still, this function is not perfect. The programmer of it chose to use pointers to prevent copying of his/her structures. Still, this code will compile:

void SomeFunction(const MyStruct1 * p1, const MyStruct2 * p2, MyStruct3 * p3)
{
  delete p1;
  delete p2;
  delete p3;
  //The rest of the function
}

In C++, however, you can use references instead.

void SomeFunction(const MyStruct1& s1, const MyStruct2& s2, MyStruct3& p3)

Note that there is no more pointer needed!

Function examples on CodePedia

C++ standard functions

From [2].

'Function' links

Code links

References

1) Bjarne Stroustrup's C++ Glossary. http://www.research.att.com/~bs/glossary.html
2) C++ International Standard. ISO/IEC 14882. Second edition. Table 99.

last edited (March 18, 2007) by bilderbikkel, Number of views: 8299, Current Rev: 22 (Diff)

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