auto (C++)

Artikel
01/03/2018

The latest version of this topic can be found at auto (C++).

Deduces the type of a declared variable from its initialization expression.

Syntax

auto declarator initializer;

[](auto param1, auto param2) {};

Remarks

The auto keyword directs the compiler to use the initialization expression of a declared variable, or lambda expression parameter, 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).

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.

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.

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;  
}

In the previous example, myAuto is an int, not an int reference, so the output is 11 11, not 11 12 as would be the case if the reference qualifier had not been dropped by auto.

Type deduction with braced initializers (C++14)

The following code exmample shows how to intialize an auto variable using braces. Note the difference between B and C and between A and E.

  
#include <initializer_list>  
  
int main()  
{  
    // std::initializer_list<int>  
    auto A = { 1, 2 };  
  
    // std::initializer_list<int>  
    auto B = { 3 };  
  
    // int  
    auto C{ 4 };  
  
    // C3535: cannot deduce type for 'auto' from initializer list'  
    auto D = { 5, 6.7 };  
  
    // C3518 in a direct-list-initialization context the type for 'auto'  
    // can only be deduced from a single initializer expression  
    auto E{ 8, 9 };  
  
    return 0;  
}

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
C3530	The `auto` keyword cannot be combined with any other type-specifier.
C3531	A symbol that is declared with the `auto` keyword must have an initializer.
C3532	You incorrectly used the `auto` keyword to declare a type. For example, you declared a method return type or an array.
C3533, C3539	A parameter or template argument cannot be declared with the `auto` keyword.
C3534	A symbol that is declared with the `auto` keyword in a `new` expression must have an initializer. For more information, see operator new.
C3535	A method or template parameter cannot be declared with the `auto` keyword.
C3536	A symbol cannot be used before it is initialized. In practice, this means that a variable cannot be used to initialize itself.
C3537	You cannot cast to a type that is declared with the `auto` keyword.
C3538	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.
C3540, C3541	The sizeof and typeid 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 const``int, 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;  
    //...  
}

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