Typename

Short description: C++ keyword

"`typename`"[1][2] is a keyword in the C++ programming language used when writing templates. It is used for specifying that a dependent name in a template definition or declaration is a type.[3][4] In the original C++ compilers before the first ISO standard was completed, the `typename` keyword was not part of the C++ language and Bjarne Stroustrup used the `class` keyword for template arguments instead. While `typename` is now the preferred keyword, older source code may still use the `class` keyword instead (for example see the difference in source code examples between The Design and Evolution of C++ by Bjarne Stroustrup published in 1994 and the source code examples in The C++ Programming Language: Fourth Edition by Bjarne Stroustrup published in 2013).

## Contents

* 1 A synonym for "class" in template parameters
* 2 A method for indicating that a dependent name is a type
* 3 See also
* 4 References

## A synonym for "`class`" in template parameters

In C++'s generic programming feature known as "templates", `typename` can be used for introducing a template parameter:[3][4]

// Define a generic function that returns the greater of its two arguments
template <typename T>
const T& max(const T& x, const T& y)

if (y < x)
return x;
return y;

An alternative and semantically equivalent keyword in this scenario is "`class`":

// Define a generic function that returns the greater of its two arguments
template <class T>
const T& max(const T& x, const T& y)

if (y < x)
return x;
return y;

## A method for indicating that a dependent name is a type

Consider this invalid code:[5][6]

template <typename T>
void foo(const T& t)

// declares a pointer to an object of type T::bar
T::bar * p;  // error (see text)

struct StructWithBarAsType

typedef int bar;

int main()

StructWithBarAsType x;
foo(x);

This code looks like it should compile, but it is incorrect because the compiler does not know if `T::bar` is a type or a value. The reason it doesn't know is that `T::bar` is a "template-parameter dependent name", or "dependent name" for short, which then could represent anything named "bar" inside a type passed to foo(), which could include typedefs, enums, variables, etc.

To resolve this ambiguity, the C++ Language Standard declares:

> A name used in a template declaration or definition and that is dependent on a template-parameter is assumed not to name a type unless the applicable name lookup finds a type name or the name is qualified by the keyword `typename`.

In short, if the compiler can't tell if a dependent name is a value or a type, then it will assume that it is a value.

In our example, where `T::bar` is the dependent name, that means that rather than declaring a pointer to `T::bar` named `p`, the line

T::bar * p;

will instead multiply the "value" `T::bar` by `p` (which is nowhere to be found) and throw away the result. The fact that in `StructWithBarAsType` the dependent `bar` is in fact a type does not help since `foo()` could be compiled long before `StructWithBarAsType` is seen. Furthermore, if there is also a class like:

struct StructWithBarAsValue

int bar;

then the compiler would be obliged to interpret the `T::bar` in `foo()` as an access to data member `StructWithBarAsValue::bar` when instantiated. But since `bar` is not a static data member it will flag an error.

The solution to this problem is to explicitly tell the compiler that `T::bar` is in fact a type. For this, the `typename` keyword is used:[3][4]

template <typename T>
void foo(const T& t)

// declares a pointer to an object of type T::bar
typename T::bar * p;

Now the compiler knows for sure that `T::bar` is a type, and will correctly make `p` a pointer to an object of that type.

## See also

* Argument-dependent name lookup – another C++ name lookup rule

## References

1. ↑ Al Stevens (April 2003). "Underdocumented C++". Dr. Dobb's Journal: 72-76.
2. ↑ T. L. Veldhuizen (2013). "C++ templates are turing complete". http://replay.waybackmachine.org/20060918135742/http://osl.iu.edu/~tveldhui/papers/2003/turing.pdf.
3. ↑ 3.0 3.1 3.2 "The typename keyword (C++ only)". IBM. http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/index.jsp?topic=/com.ibm.xlcpp8a.doc/language/ref/keyword_typename.htm. Retrieved 23 August 2013.
4. ↑ 4.0 4.1 4.2 "MSDN - typename". MSDN. http://msdn.microsoft.com/en-us/library/8y88s595.aspxtypename. Retrieved 23 August 2013. [yes|permanent dead link|dead link}}]
5. ↑ "Dependent name lookup for C++ templates". February 6, 2012. https://eli.thegreenplace.net/2012/02/06/dependent-name-lookup-for-c-templates.
6. ↑ "Types-, Non-Types, and Templates as Template Parameters". March 4, 2019. https://www.modernescpp.com/index.php/types-non-types-and-templates-as-template-parameters.

0.00

(0 votes)

Original source: https://en.wikipedia.org/wiki/Typename. Read more

|

Retrieved from "https://handwiki.org/wiki/index.php?title=Typename&oldid=83329"