auto Keyword (Type Deduction)
Deduces the type of a declared variable from its initialization expression.
auto declarator initializer;
Remarks
The auto keyword directs the compiler to use the initialization expression of a declared variable to deduce its type.
We recommend that you use the auto keyword for most situations—unless you really want a conversion—because it provides these benefits:
Robustness: If the expression’s type is changed—this includes when a function return type is changed—it just works.
Performance: You’re guaranteed that there will be no conversion.
Usability: You don't have to worry about type name spelling difficulties and typos.
Efficiency: Your coding can be more efficient.
Conversion cases in which you might not want to use auto:
When you want a specific type and nothing else will do.
Expression template helper types—for example, (valarray+valarray) and initializer lists—although rarely would you choose to write auto x = { 1 }; and really expect to get an int.
To use the auto keyword, use it instead of a type to declare a variable, and specify an initialization expression. In addition, you can modify the auto keyword by using specifiers and declarators such as const, volatile, pointer (*), reference (&), and rvalue reference (&&). The compiler evaluates the initialization expression and then uses that information to deduce the type of the variable.
The initialization expression can be an assignment (equal-sign syntax), a direct initialization (function-style syntax), an operator new expression, or the initialization expression can be the for-range-declaration parameter in a Range-based for Statement (C++) statement. For more information, see Initializers and the code examples later in this document.
The auto keyword is a placeholder for a type, but it is not itself a type. Therefore, the auto keyword cannot be used in casts or operators such as sizeof and typeid (C++ Component Extensions).
Usefulness
The auto keyword is a simple way to declare a variable that has a complicated type. For example, you can use auto to declare a variable where the initialization expression involves templates, pointers to functions, or pointers to members.
You can also use auto to declare and initialize a variable to a lambda expression. You can't declare the type of the variable yourself because the type of a lambda expression is known only to the compiler. For more information, see Examples of Lambda Expressions.
Trailing Return Types
You can use auto, together with the decltype type specifier, to help write template libraries. Use auto and decltype to declare a template function whose return type depends on the types of its template arguments. Or, use auto and decltype to declare a template function that wraps a call to another function, and then returns whatever is the return type of that other function. For more information, see decltype Type Specifier.
References and cv-qualifiers
Note that using auto drops references, const qualifiers, and volatile qualifiers. Consider the following example:
// cl.exe /analyze /EHsc /W4
#include <iostream>
using namespace std;
int main( )
{
int count = 10;
int& countRef = count;
auto myAuto = countRef;
countRef = 11;
cout << count << " ";
myAuto = 12;
cout << count << endl;
}
You may believe myAuto is an int reference, but it is not. It is simply an int, so the output is 11 11, not 11 12 as would be the case if the reference had not been dropped by auto.
Restrictions and Error Messages
The following table lists the restrictions on the use of the auto keyword, and the corresponding diagnostic error message that the compiler emits.
Error number |
Description |
|---|---|
The auto keyword cannot be combined with any other type-specifier. |
|
A symbol that is declared with the auto keyword must have an initializer. |
|
You incorrectly used the auto keyword to declare a type. For example, you declared a method return type or an array. |
|
A parameter or template argument cannot be declared with the auto keyword. |
|
A symbol that is declared with the auto keyword in a new expression must have an initializer. For more information, see operator new (<new>). |
|
A method or template parameter cannot be declared with the auto keyword. |
|
A symbol cannot be used before it is initialized. In practice, this means that a variable cannot be used to initialize itself. |
|
You cannot cast to a type that is declared with the auto keyword. |
|
All the symbols in a declarator list that is declared with the auto keyword must resolve to the same type. For more information, see Declarations. |
|
The sizeof and typeid (C++ Component Extensions) operators cannot be applied to a symbol that is declared with the auto keyword. |
Examples
These code fragments illustrate some of the ways in which the auto keyword can be used.
The following declarations are equivalent. In the first statement, variable j is declared to be type int. In the second statement, variable k is deduced to be type int because the initialization expression (0) is an integer.
int j = 0; // Variable j is explicitly type int.
auto k = 0; // Variable k is implicitly type int because 0 is an integer.
The following declarations are equivalent, but the second declaration is simpler than the first. One of the most compelling reasons to use the auto keyword is simplicity.
map<int,list<string>>::iterator i = m.begin();
auto i = m.begin();
The following code fragment declares the type of variables iter and elem when the for and range for loops start.
// cl /EHsc /nologo /W4
#include <deque>
using namespace std;
int main()
{
deque<double> dqDoubleData(10, 0.1);
for (auto iter = dqDoubleData.begin(); iter != dqDoubleData.end(); ++iter)
{ /* ... */ }
// prefer range-for loops with the following information in mind
// (this applies to any range-for with auto, not just deque)
for (auto elem : dqDoubleData) // COPIES elements, not much better than the previous examples
{ /* ... */ }
for (auto& elem : dqDoubleData) // observes and/or modifies elements IN-PLACE
{ /* ... */ }
for (const auto& elem : dqDoubleData) // observes elements IN-PLACE
{ /* ... */ }
}
The following code fragment uses the new operator and pointer declaration to declare pointers.
double x = 12.34;
auto *y = new auto(x), **z = new auto(&x);
The next code fragment declares multiple symbols in each declaration statement. Notice that all of the symbols in each statement resolve to the same type.
auto x = 1, *y = &x, **z = &y; // Resolves to int.
auto a(2.01), *b (&a); // Resolves to double.
auto c = 'a', *d(&c); // Resolves to char.
auto m = 1, &n = m; // Resolves to int.
This code fragment uses the conditional operator (?:) to declare variable x as an integer that has a value of 200:
int v1 = 100, v2 = 200;
auto x = v1 > v2 ? v1 : v2;
The following code fragment initializes variable x to type int, variable y to a reference to type constint, and variable fp to a pointer to a function that returns type int.
int f(int x) { return x; }
int main()
{
auto x = f(0);
const auto & y = f(1);
int (*p)(int x);
p = f;
auto fp = p;
//...
}
See Also
Reference
/Zc:auto (Deduce Variable Type)
typeid (C++ Component Extensions)