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C++ difference of keywords ‘typename’ and ‘class’ in templates
When defining a function template or class template in C++, one can write this:
template <class T> ...
or one can write this:
template <typename T> ...
Is there a good reason to prefer one over the other?
I accepted the most popular (and interesting) answer, but the real answer seems to be "No, there is no good reason to prefer one over the other."
typename.class.Note, however, that before C++17 in the case of template template parameters, use of class instead of typename was required. See user1428839's answer below. (But this particular case is not a matter of preference, it was a requirement of the language.)
I prefer to use typename because I'm not a fan of overloaded keywords (jeez - how many different meanings does static have for various different contexts?).
It doesn't matter at all, but class makes it look like T can only be a class, while it can of course be any type. So typename is more accurate. On the other hand, most people use class, so that is probably easier to read generally.
Extending DarenW's comment.
Once typename and class are not accepted to be very different, it might be still valid to be strict on their use. Use class only if is really a class, and typename when its a basic type, such as char.
These types are indeed also accepted instead of typename
template< char myc = '/' >
which would be in this case even superior to typename or class.
Think of "hintfullness" or intelligibility to other people. And actually consider that 3rd party software/scripts might try to use the code/information to guess what is happening with the template (consider swig).
It doesn't matter at all, but class makes it look like T can only be a class, while it can of course be any type. So typename is more accurate. On the other hand, most people use class, so that is probably easier to read generally.
As far as I know, it doesn't matter which one you use. They're equivalent in the eyes of the compiler. Use whichever one you prefer. I normally use class.
It doesn't matter at all, but class makes it look like T can only be a class, while it can of course be any type. So typename is more accurate. On the other hand, most people use class, so that is probably easier to read generally.
class to typename.typename is illegal for template template arguments, so to be consistent, you should use class:
template<template<class> typename MyTemplate, class Bar> class Foo { }; // :(
template<template<class> class MyTemplate, class Bar> class Foo { }; // :)
I prefer to use typename because I'm not a fan of overloaded keywords (jeez - how many different meanings does static have for various different contexts?).
According to Scott Myers, Effective C++ (3rd ed.) item 42 (which must, of course, be the ultimate answer) - the difference is "nothing".
Advice is to use "class" if it is expected T will always be a class, with "typename" if other types (int, char* whatever) may be expected. Consider it a usage hint.
Just pure history. Quote from Stan Lippman:
The reason for the two keywords is historical. In the original template specification, Stroustrup reused the existing class keyword to specify a type parameter rather than introduce a new keyword that might of course break existing programs. It wasn't that a new keyword wasn't considered -- just that it wasn't considered necessary given its potential disruption. And up until the ISO-C++ standard, this was the only way to declare a type parameter.
But one should use typename rather than class! See the link for more info, but think about the following code:
template <class T>
class Demonstration {
public:
void method() {
T::A *aObj; // oops ...
};
According to Scott Myers, Effective C++ (3rd ed.) item 42 (which must, of course, be the ultimate answer) - the difference is "nothing".
Advice is to use "class" if it is expected T will always be a class, with "typename" if other types (int, char* whatever) may be expected. Consider it a usage hint.
Just pure history. Quote from Stan Lippman:
The reason for the two keywords is historical. In the original template specification, Stroustrup reused the existing class keyword to specify a type parameter rather than introduce a new keyword that might of course break existing programs. It wasn't that a new keyword wasn't considered -- just that it wasn't considered necessary given its potential disruption. And up until the ISO-C++ standard, this was the only way to declare a type parameter.
But one should use typename rather than class! See the link for more info, but think about the following code:
template <class T>
class Demonstration {
public:
void method() {
T::A *aObj; // oops ...
};
class to typename.typename is illegal for template template arguments, so to be consistent, you should use class:
template<template<class> typename MyTemplate, class Bar> class Foo { }; // :(
template<template<class> class MyTemplate, class Bar> class Foo { }; // :)
According to Scott Myers, Effective C++ (3rd ed.) item 42 (which must, of course, be the ultimate answer) - the difference is "nothing".
Advice is to use "class" if it is expected T will always be a class, with "typename" if other types (int, char* whatever) may be expected. Consider it a usage hint.
In response to Mike B, I prefer to use 'class' as, within a template, 'typename' has an overloaded meaning, but 'class' does not. Take this checked integer type example:
template <class IntegerType>
class smart_integer {
public:
typedef integer_traits<Integer> traits;
IntegerType operator+=(IntegerType value){
typedef typename traits::larger_integer_t larger_t;
larger_t interm = larger_t(myValue) + larger_t(value);
if(interm > traits::max() || interm < traits::min())
throw overflow();
myValue = IntegerType(interm);
}
}
larger_integer_t is a dependent name, so it requires 'typename' to preceed it so that the parser can recognize that larger_integer_t is a type. class, on the otherhand, has no such overloaded meaning.
That... or I'm just lazy at heart. I type 'class' far more often than 'typename', and thus find it much easier to type. Or it could be a sign that I write too much OO code.
As an addition to all above posts, the use of the class keyword is forced (up to and including C++14) when dealing with template template parameters, e.g.:
template <template <typename, typename> class Container, typename Type>
class MyContainer: public Container<Type, std::allocator<Type>>
{ /*...*/ };
In this example, typename Container would have generated a compiler error, something like this:
error: expected 'class' before 'Container'
As far as I know, it doesn't matter which one you use. They're equivalent in the eyes of the compiler. Use whichever one you prefer. I normally use class.
It doesn't matter at all, but class makes it look like T can only be a class, while it can of course be any type. So typename is more accurate. On the other hand, most people use class, so that is probably easier to read generally.
I prefer to use typename because I'm not a fan of overloaded keywords (jeez - how many different meanings does static have for various different contexts?).
As far as I know, it doesn't matter which one you use. They're equivalent in the eyes of the compiler. Use whichever one you prefer. I normally use class.
In response to Mike B, I prefer to use 'class' as, within a template, 'typename' has an overloaded meaning, but 'class' does not. Take this checked integer type example:
template <class IntegerType>
class smart_integer {
public:
typedef integer_traits<Integer> traits;
IntegerType operator+=(IntegerType value){
typedef typename traits::larger_integer_t larger_t;
larger_t interm = larger_t(myValue) + larger_t(value);
if(interm > traits::max() || interm < traits::min())
throw overflow();
myValue = IntegerType(interm);
}
}
larger_integer_t is a dependent name, so it requires 'typename' to preceed it so that the parser can recognize that larger_integer_t is a type. class, on the otherhand, has no such overloaded meaning.
That... or I'm just lazy at heart. I type 'class' far more often than 'typename', and thus find it much easier to type. Or it could be a sign that I write too much OO code.
As far as I know, it doesn't matter which one you use. They're equivalent in the eyes of the compiler. Use whichever one you prefer. I normally use class.
Extending DarenW's comment.
Once typename and class are not accepted to be very different, it might be still valid to be strict on their use. Use class only if is really a class, and typename when its a basic type, such as char.
These types are indeed also accepted instead of typename
template< char myc = '/' >
which would be in this case even superior to typename or class.
Think of "hintfullness" or intelligibility to other people. And actually consider that 3rd party software/scripts might try to use the code/information to guess what is happening with the template (consider swig).
According to Scott Myers, Effective C++ (3rd ed.) item 42 (which must, of course, be the ultimate answer) - the difference is "nothing".
Advice is to use "class" if it is expected T will always be a class, with "typename" if other types (int, char* whatever) may be expected. Consider it a usage hint.
In response to Mike B, I prefer to use 'class' as, within a template, 'typename' has an overloaded meaning, but 'class' does not. Take this checked integer type example:
template <class IntegerType>
class smart_integer {
public:
typedef integer_traits<Integer> traits;
IntegerType operator+=(IntegerType value){
typedef typename traits::larger_integer_t larger_t;
larger_t interm = larger_t(myValue) + larger_t(value);
if(interm > traits::max() || interm < traits::min())
throw overflow();
myValue = IntegerType(interm);
}
}
larger_integer_t is a dependent name, so it requires 'typename' to preceed it so that the parser can recognize that larger_integer_t is a type. class, on the otherhand, has no such overloaded meaning.
That... or I'm just lazy at heart. I type 'class' far more often than 'typename', and thus find it much easier to type. Or it could be a sign that I write too much OO code.
I prefer to use typename because I'm not a fan of overloaded keywords (jeez - how many different meanings does static have for various different contexts?).
As an addition to all above posts, the use of the class keyword is forced (up to and including C++14) when dealing with template template parameters, e.g.:
template <template <typename, typename> class Container, typename Type>
class MyContainer: public Container<Type, std::allocator<Type>>
{ /*...*/ };
In this example, typename Container would have generated a compiler error, something like this:
error: expected 'class' before 'Container'
Source: Stackoverflow.com