Declaring and initializing a variable in a Conditional or Control statement in C++

Question

In Stroustrup\'s The C++ Programming Language: Special Edition (3rd Ed), Stroustrup writes that the declaration and initialization of variables in the conditionals

Answer 1

It is allowed to declare a variable in the control part of a nested block, but in the case of if and while, the variable must be initialized to a numeric or boolean value that will be interpreted as the condition. It cannot be included in a more complex expression!

In the particular case you show, it doesn't seem you can find a way to comply unfortunately.

I personally think it's good practice to keep the local variables as close as possible to their actual lifetime in the code, even if that sounds shocking when you switch from C to C++ or from Pascal to C++ - we were used to see all the variables at one place. With some habit, you find it more readable, and you don't have to look elsewhere to find the declaration. Moreover, you know that it is not used before that point.

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Edit:

That being said, I don't find it a good practice to mix too much in a single statement, and I think it's a shared opinion. If you affect a value to a variable, then use it in another expression, the code will be more readable and less confusing by separating both parts.

So rather than using this:

int i;
if((i = read(socket)) < 0) {
    // handle error
}
else if(i > 0) {
    // handle input
}
else {
    return true;
}

I would prefer that:

int i = read(socket);
if(i < 0) {
    // handle error
}
else if(i > 0) {
    // handle input
}
else {
    return true;
}

Answer 2

While you can use a declaration as a boolean expression, you cannot place a declaration in the middle of an expression. I cannot help thinking that you are mis-reading what Bjarne is saying.

The technique is useful and desirable mostly for control variables of for loops, but in this instance I believe is ill-advised and does not serve clarity. And of course it does not work! ;)

if( <type> <identifier> = <initialiser> ) // valid, but not that useful IMO

if( (<type> <identifier> = <initialiser>) <operator> <operand> )  // not valid

for( <type> <identifier> = <initialiser>; 
     <expression>; 
     <expression> )  // valid and desirable

In your example you have called a function with side effects in a conditional, which IMO is a bad idea regardless of what you might think about declaring the variable there.

Answer 3

Adding to what RedGlyph and Ferruccio said. May be we can do the following to still declare within a conditional statement to limit its use:

if(int x = read(socket)) //x != 0
{
  if(x < 0) //handle error
  {}
  else //do work
  {}
}
else //x == 0  
{
  return true;
}

Answer 4

To complement the other folks' good answers, you can always limit the scope of the variable by braces:

{    
  const int readResult = read(socket);
  if(readResult < 0) {
    // handle error
  } 
  else if(readResult > 0)
  {
    // handle input
  } 
  else {
    return true;
  } 
}

Answer 5

While not directly related to the question, all of the examples put the error handling first. Since there are 3 cases (>0 -> data, ==0 -> connection closed and <0 -> error), that means that the most common case of getting new data requires two tests. Checking for >0 first would cut the expected number of tests by almost half. Unfortunately the "if(int x = read(socket))" approach given by White_Pawn still requires 2 tests for the case of data, but the C++17 proposal could be used to test for >0 first.

Answer 6

I use const as much as possible in these situations. Instead of your example, I would do:

const int readResult = read(socket);
if(readResult < 0) {
    // handle error
} 
else if(readResult > 0)
{
    // handle input
} 
else {
    return true;
} 

So although the scope isn't contained, it doesn't really matter, since the variable can't be altered.