Incorrect Multiple Cases in Switch not generating compiler error

This question already has an answer here:

What does comma operator mean in a switch statement? 6 answers

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Just looking quickly at this code, we think the return value should be 1,

I'd say that an experienced C++ developer would immediately notice that something is wrong and quickly conclude that some other programmer accidentally tried to use the comma operator: ,

but in the execution it returns returns 3.

No, the code must not compile , because the case expression is not constant

And as a matter of fact, it does not compile in any halfway modern compiler. For example, MSVC 2013 says:

stackoverflow.cpp(8) : error C2051: case expression not constant
stackoverflow.cpp(10) : error C2051: case expression not constant

An expression like 1, 2 is an application of the comma operator, and the comma operator implies that the expression is not a compile-time constant.

At least until C++11 came along and relaxed the rules to the effect that adding parentheses, ie case (1, 2): , would be allowed to compile. It just would not do what you seem to expect.

This is partially answered in Multiple Cases in Switch:

How so? That other question and the answers are almost exclusively about C#, not about C++.

I would like to know why the incorrect form does compile without error or event warnings (at least in the Borland C++ compiler).

Because the compiler is too old. Better get a new one.

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My guess is that in the first case the compiler evaluates the comma operators to result in the code being executed as follows:

switch(variable)
{
  case 2:
    return 1;
  case 4:
    return 2;
  default:
    return 3;
}

From above, it can be seen why the value 3 is returned for input 1. I suggest you read up about the comma operator. There are some excellent threads on SO related to it.

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a, b is a valid expression in both C and C++. It means "Evaluate a , discard it, evaluate b ". The value of the expression is b . So your original switch has the following meaning:

variable = 1;
switch(variable)
{
  case 2:
    return 1;
  case 4:
    return 2;
  default:
    return 3;
}

For more on the comma operator, you can read the Wikipedia article.

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