C++ Operator Precedence [C++ Reference]
Keywords: operator_precedence
Date: 2010-04-25T03:13:27-0700

## C++ Operator Precedence

The operators at the top of this list are evaluated first. Operators within a group have the same precedence. All operators have left-to-right associativity unless otherwise noted.

Precedence | Operator | Description | Example | Overloadable | Associativity
--- | --- | --- | --- | --- | ---
1 | `::` | Scope resolution operator | Class::age = 2; | no | left to right
2 | `()` `()` `[]` `->` `.` `++` `--` `dynamic_cast` `static_cast` `reinterpret_cast` `const_cast` `typeid` | Function call Member initalization Array access Member access from a pointer Member access from an object Post-increment Post-decrement Runtime-checked type conversion Unchecked type conversion Reinterpreting type conversion Cast away/Add constness Get type information | isdigit('1') c_tor(int x, int y) : _x(x), _y(y * 10) {} array[4] = 2; ptr->age = 34; obj.age = 34; for (int i = 0; i < 10; i++) cout << i; for (int i = 10; i > 0; i--) cout << i; Y& y = dynamic_cast<Y&>(x); Y& y = static_cast<Y&>(x); int const* p = reinterpret_cast<int const*>(0x1234); int* q = const_cast<int*>(p); std::type_info const& t = typeid(x); | yes yes yes yes no yes yes no no no no no | left to right
3 | `!` `not` `~` `compl` `++` `--` `-` `+` `*` `&` `sizeof` `new` `new []` `delete` `delete []` `(type)` | Logical negation Alternate spelling for ! Bitwise complement Alternate spelling for ~ Pre-increment Pre-decrement Unary minus Unary plus Dereference Address of Size (of the type) of the operand in bytes Dynamic memory allocation Dynamic memory allocation of array Deallocating the memory Deallocating the memory of array Cast to a given type | if (!done) … flags = ~flags; for (i = 0; i < 10; ++i) cout << i; for (i = 10; i > 0; --i) cout << i; int i = -1; int i = +1; int data = *intPtr; int *intPtr = &data; size_t s = sizeof(int); long* pVar = new long; long* array = new long[20]; delete pVar; delete [] array; int i = (int)floatNum; | yes yes yes yes yes yes yes yes no yes yes yes yes yes | right to left
4 | `->*` `.*` | Member pointer selector Member object selector | ptr->*var = 24; obj.*var = 24; | yes no | left to right
5 | `*` `/` `%` | Multiplication Division Modulus | int i = 2 * 4; float f = 10.0 / 3.0; int rem = 4 % 3; | yes yes yes | left to right
6 | `+` `-` | Addition Subtraction | int i = 2 + 3; int i = 5 - 1; | yes yes | left to right
7 | `<<` `>>` | Bitwise shift left Bitwise shift right | int flags = 33 << 1; int flags = 33 >> 1; | yes yes | left to right
8 | `<` `<=` `>` `>=` | Comparison less-than Comparison less-than-or-equal-to Comparison greater-than Comparison greater-than-or-equal-to | if (i < 42) … if (i <= 42) ... if (i > 42) … if (i >= 42) ... | yes yes yes yes | left to right
9 | `==` `eq` `!=` `not_eq` | Comparison equal-to Alternate spelling for == Comparison not-equal-to Alternate spelling for != | if (i == 42) ... if (i != 42) … | yes - yes | left to right
10 | `&` `bitand` | Bitwise AND Alternate spelling for & | flags = flags & 42; | yes | left to right
11 | `^` `xor` | Bitwise exclusive OR (XOR) Alternate spelling for ^ | flags = flags ^ 42; | yes | left to right
12 | `|` `bitor` | Bitwise inclusive (normal) OR Alternate spelling for | | flags = flags | 42; | yes | left to right
13 | `&&` `and` | Logical AND Alternate spelling for && | if (conditionA && conditionB) … | yes | left to right
14 | `||` `or` | Logical OR Alternate spelling for || | if (conditionA || conditionB) ... | yes | left to right
15 | `? :` | Ternary conditional (if-then-else) | int i = a > b ? a : b; | no | right to left
16 | `=` `+=` `-=` `*=` `/=` `%=` `&=` `and_eq` `^=` `xor_eq` `|=` `or_eq` `<<=` `>>=` | Assignment operator Increment and assign Decrement and assign Multiply and assign Divide and assign Modulo and assign Bitwise AND and assign Alternate spelling for &= Bitwise exclusive or (XOR) and assign Alternate spelling for ^= Bitwise normal OR and assign Alternate spelling for |= Bitwise shift left and assign Bitwise shift right and assign | int a = b; a += 3; b -= 4; a *= 5; a /= 2; a %= 3; flags &= new_flags; flags ^= new_flags; flags |= new_flags; flags <<= 2; flags >>= 2; | yes yes yes yes yes yes yes yes yes yes yes | right to left
17 | `throw` | throw exception | throw EClass(“Message”); | no |
18 | `,` | Sequential evaluation operator | for (i = 0, j = 0; i < 10; i++, j++) … | yes | left to right

## Order of Evaluation and of Side Effects

One important aspect of C++ that is related to operator precedence, is the order of evaluation and the order of side effects in expressions. In most circumstances, the order in which things happen is not specified. For example in `f() + g()` whether `f()` or `g()` is called first is not specified. If at least one of the functions has side effects the results may differ across compilers, different versions of the same compiler or even between multiple runs of the same compiler.

Further, the effect of certain expressions is undefined. For example, consider the following code:

        float x = 1;
        x = x / ++x;

The value of x and the rest of the behaviour of the program after evaluating this expression is undefined. The program is semantically ill-formed: x is modified twice between two consecutive sequence points.

Expressions like the one above must be avoided. When in doubt, break a large expression into multiple statements to ensure that the order of evaluation is correct.

## Overloading of Operators

Overloading of operators can be very useful and very dangerous. On one hand overloading operators for a class you have created can help with logistics and readability of code. On the other hand you can overload an operator in such a way that it can either obfuscate or just downright break your program. Use carefully. In particular never overload `&&`, `||` or `,`. In the overloaded context they lose the guarantee that the left operand is evaluated before the second and that there is a sequence point inbetween.

There are two ways to over load an operator: global function or class member.

Example of overloading with a global function:

         ostream& operator <<(ostream& os, const myClass& rhs);

But to be able to reach any private data within a user defined class you have to declare the global function as a friend within the definition of the class.

Example:

         class myClass {

           // Gives the operator << function access to 'myData'
           // (this declaration should not go in public, private or protected)
           friend ostream& operator <<(ostream& lhs, const myClass& rhs);

           private:
             int myData;
         }

Overloading with a class member can be done as follows:

         class myClass {

           public:
             // The left hand side of this operator becomes '*this'.
             int operator +(const myClass& rhs);

           private:
             int myData;
         }