The assignment operator assigns a value to a variable.
a = 5;
This statement assigns the integer value 5 to the variable a. The part at the left of the assignment operator (=) is
known as the lvalue (left value) and the right one as the rvalue (right value). The lvalue has to be a variable
whereas the rvalue can be either a constant, a variable, the result of an operation or any combination of these.
The most important rule when assigning is the right-to-left rule: The assignment operation always takes place from
right to left, and never the other way:
a = b;
This statement assigns to variable a (the lvalue) the value contained in variable b (the rvalue). The value that was
stored until this moment in a is not considered at all in this operation, and in fact that value is lost.
Consider also that we are only assigning the value of b to a at the moment of the assignment operation. Therefore
a later change of b will not affect the new value of a.
For example, let us have a look at the following code - I have included the evolution of the content stored in the
variables as comments:
// assignment operator
#include <iostream>
using namespace std;
int main ()
{
int a, b; // a:?, b:?
a = 10; // a:10, b:?
b = 4; // a:10, b:4
a = b; // a:4, b:4
b = 7; // a:4, b:7
cout << "a:";
cout << a;
cout << " b:";
cout << b;
return 0;
}
a:4 b:7
This code will give us as result that the value contained in a is 4 and the one contained in b is 7. Notice how a was
not affected by the final modification of b, even though we declared a = b earlier (that is because of the right-toleft
rule).
A property that C++ has over other programming languages is that the assignment operation can be used as the
rvalue (or part of an rvalue) for another assignment operation. For example:
a = 2 + (b = 5);
is equivalent to:
b = 5;
a = 2 + b;
that means: first assign 5 to variable b and then assign to a the value 2 plus the result of the previous assignment
of b (i.e. 5), leaving a with a final value of 7.
The following expression is also valid in C++:
a = b = c = 5;
It assigns 5 to the all the three variables: a, b and c.
a = 5;
This statement assigns the integer value 5 to the variable a. The part at the left of the assignment operator (=) is
known as the lvalue (left value) and the right one as the rvalue (right value). The lvalue has to be a variable
whereas the rvalue can be either a constant, a variable, the result of an operation or any combination of these.
The most important rule when assigning is the right-to-left rule: The assignment operation always takes place from
right to left, and never the other way:
a = b;
This statement assigns to variable a (the lvalue) the value contained in variable b (the rvalue). The value that was
stored until this moment in a is not considered at all in this operation, and in fact that value is lost.
Consider also that we are only assigning the value of b to a at the moment of the assignment operation. Therefore
a later change of b will not affect the new value of a.
For example, let us have a look at the following code - I have included the evolution of the content stored in the
variables as comments:
// assignment operator
#include <iostream>
using namespace std;
int main ()
{
int a, b; // a:?, b:?
a = 10; // a:10, b:?
b = 4; // a:10, b:4
a = b; // a:4, b:4
b = 7; // a:4, b:7
cout << "a:";
cout << a;
cout << " b:";
cout << b;
return 0;
}
a:4 b:7
This code will give us as result that the value contained in a is 4 and the one contained in b is 7. Notice how a was
not affected by the final modification of b, even though we declared a = b earlier (that is because of the right-toleft
rule).
A property that C++ has over other programming languages is that the assignment operation can be used as the
rvalue (or part of an rvalue) for another assignment operation. For example:
a = 2 + (b = 5);
is equivalent to:
b = 5;
a = 2 + b;
that means: first assign 5 to variable b and then assign to a the value 2 plus the result of the previous assignment
of b (i.e. 5), leaving a with a final value of 7.
The following expression is also valid in C++:
a = b = c = 5;
It assigns 5 to the all the three variables: a, b and c.