Prevent a returning function from execution if a condition on parameters is true

Question

like said on the title, I was wondering if it was possible to stop a function from execution. In my particular case, I'm trying to make an operator[] and prevent utilisator from using it if the value gave in parameters is too high :

in .h:

class Vec4
{
    float x,y,z,w;

    public:

        float operator[](const unsigned int i);
}

in .cpp :

float Vec4::operator[](const unsigned int i)
{
    if(i == 0) return x;
    if(i == 1) return y;
    if(i == 2) return z;
    if(i == 3) return w;
}

I'd like to "break" the function if i >=4 For the moment I'm just making a console output and return 0.0f

thank you to show me if there is a way ... or not !

Answer 1

You can do at least 4 things.

1. Return a known error value from the function. For eg. -1 if the input value is no good.
2. Raise an exception.
3. Change the function to pass output by reference and return an error code.
4. Force the user to get the point with a strongly typed enum class.

Option 1

float Vec4::operator[](const unsigned int i) {
    switch (i)
    case 0: 
      return x;
    ...
    default:
        return nan;

Option 2

default:
    throw InvalidInputException;

Option 3

typedef ErrCode int;
const int ERROR = -1;
const int SUCCESS = 1;
...
ErrCode Vec4::getPoint(const unsigned int i, float &ouptut) {
    ...
    switch (i)
    case 0: 
      output = x;
      return SUCCESS;
    default:
      return ERROR;

Option 4 (c++11)

class Vec4 {
...
public:
    enum class VecMem {X, Y, Z, W};
    float Vec4::getPoint(VecMem member) {
        switch (member):
            case X:
                return x;
        ...

Usage:

Vec4.getPoint(Vec4::VecMem::X);