auto
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The auto keyword has been redefined in the C++11 and C++17 standards. Thus, C++Builder supports different definitions of auto, determined by the standard that each C++ compiler follows.
- The C++98 Definition is supported by previous-generation C++ compilers (BCC32).
- The C++11 Definition is supported by C++11-compatible Clang-enhanced C++ compilers.
- The C++14 and C++17 Definitions are supported by C++11-compatible Clang-enhanced C++ compilers.
This page describes the definitions of auto.
Contents
C++98 Definition
Category
Storage Class Specifiers (C++)
Syntax
[auto] <data-definition> ;
Description
Use the auto modifier to define a local variable as having a local lifetime.
This is the default for local variables and is rarely used.
Example:
void f() {
auto int x;
}
C++11 Definition
Category
Syntax
[auto] <variable_name> = <initializer_expression> ;
Description
Use auto as a simple type specifier that deduces its semantics from the initializer expression.
Example:
int f() {
return 1;
}
int _tmain(int argc, _TCHAR* argv[]) {
auto x = f();
return 0;
}
In this example, the type of the variable x is deduced from its initializer expression: f(). Therefore, x would be of type int, the type returned by f().
C++14 definition
In C++14, auto can be used as a return type for a method, deducing the return type, as well as for type inference for a variable or constant, as in C++11.
Example:
// C++14 introduces using the auto keyword for a return type
auto MyMethod() {
return “Hello world”;
}
int _tmain(int argc, _TCHAR* argv[]) {
// C++11 introduced type deduction for a variable / constant
auto x = MyMethod();
return 0;
}
C++17 Definition
In C++17, auto can be used in a template parameter to deduce the type, and for structured bindings.
Structured bindings allow you to easily tie individual variables to elements of a tuple. This means a method using std::tuple to effectively return multiple return values can have those values assigned to multiple individual variables in the calling method.
Example:
std::tuple<int, float> returnMultiple()
{ int i = 1; float f = 3.14; return std::make_tuple(i, f); }
// In the calling method, with i and f predefined variables:
auto [i, f] = returnMultiple();
Code Migration Issue
When Clang-enhanced C++ compilers encounter old auto syntax (that is, auto used as a storage class specifier), they raise the following warning:
[<compiler> Warning] File1.cpp(12): 'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases.