Using assignment as a condition expression?

by

C++ — ISO/IEC 14882:2003(E)

[5.17/1] There are several assignment operators, all of which group
right-to-left. All require a modifiable lvalue as their left operand,
and the type of an assignment expression is that of its left operand.
The result of the assignment operation is the value stored in the left
operand after the assignment has taken place; the result is an lvalue.

The result of the expression a = 5 is 5.

[6.4/4] [..] The value of a condition that is an expression is the value of the
expression, implicitly converted to bool for statements other than
switch. [..]

A conversion to bool takes place.

[4.12/1] An rvalue of arithmetic, enumeration, pointer, or pointer to member
type can be converted to an rvalue of type bool. A zero value, null
pointer value, or null member pointer value is converted to false; any
other value is converted to true.

5 converts to boolean true.

[6.4.1/1] If the condition (6.4) yields true the first
substatement is executed. [..]

true is treated as an if statement success.

C — ISO/IEC 9899:1999(E)

[6.5.16/3] An assignment operator stores a value in the object
designated by the left operand. An assignment expression has the value
of the left operand after the assignment, but is not an lvalue. [..]

The result of the expression a = 5 is 5.

[6.8.4.1/2] In both forms, the first substatement is executed if the
expression compares unequal to 0. [..]

5 is treated as an if statement success.

General

Code like this is almost always a mistake; the author likely intended if (a == 5) {}. However, sometimes it is deliberate. You may see code like this:

if (x = foo()) {
   cout << "I set x to the result of foo(), which is truthy";
   // ... stuff
}

More Related Contents:

  1. If statements in for loops
  2. If statement – C++
  3. if-else statement only works in a specific order and I don’t know why [closed]
  4. C++ use if else
  5. What is the copy-and-swap idiom?
  6. Most efficient way to compare a variable to multiple values?
  7. Is (4 > y > 1) a valid statement in C++? How do you evaluate it if so?
  8. Why must the copy assignment operator return a reference/const reference?
  9. if (cin >> x) – Why can you use that condition?
  10. C++, variable declaration in ‘if’ expression
  11. How to use base class’s constructors and assignment operator in C++?
  12. Checking for NULL pointer in C/C++ [closed]
  13. Math-like chaining of the comparison operator – as in, “if ( (5
  14. If always returns true [duplicate]
  15. Why should the assignment operator return a reference to the object?
  16. How to avoid “if” chains?
  17. The copy constructor and assignment operator
  18. Benefits of ternary operator vs. if statement
  19. How are C++ array members handled in copy control functions?
  20. Why cannot a non-member function be used for overloading the assignment operator?
  21. Defining a variable in the condition part of an if-statement?
  22. Overloading assignment operator in C++
  23. Explicit copy constructor
  24. Setting extra bits in a bool makes it true and false at the same time
  25. C++ why the assignment operator should return a const ref in order to avoid (a=b)=c
  26. Should the Copy-and-Swap Idiom become the Copy-and-Move Idiom in C++11?
  27. if statement not working right?
  28. Do I need to put constexpr after else-if?
  29. if / else at compile time in C++?
  30. C++ copy-construct construct-and-assign question

Leave a Comment