Substitution failure is not an error: Difference between revisions

Revision as of 12:37, 31 August 2019

Revision as of 18:35, 11 May 2020

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The following example illustrates a basic instance of SFINAE:

<sourcesyntaxhighlight lang="cpp">

struct Test {

typedef int foo;

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// thanks to SFINAE.

}

</syntaxhighlight>

</source>

Here, attempting to use a non-class type in a qualified name (<code>T::foo</code>) results in a deduction failure for <code>f<int></code> because <code>int</code> has no nested type named <code>foo</code>, but the program is well-formed because a valid function remains in the set of candidate functions.

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For example, SFINAE can be used to determine if a type contains a certain typedef:

<sourcesyntaxhighlight lang="cpp">

#include <iostream>

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std::cout << has_typedef_foobar<foo>::value << std::endl; // Prints true

}

</syntaxhighlight>

</source>

When <code>T</code> has the nested type <code>foobar</code> defined, the instantiation of the first <code>test</code> works and the null pointer constant is successfully passed. (And the resulting type of the expression is <code>yes</code>.) If it does not work, the only available function is the second <code>test</code>, and the resulting type of the expression is <code>no</code>. An ellipsis is used not only because it will accept any argument, but also because its conversion rank is lowest, so a call to the first function will be preferred if it is possible; this removes ambiguity.

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In [[C++11]], the above code could be simplified to:

<sourcesyntaxhighlight lang="cpp">

#include <iostream>

#include <type_traits>

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std::cout << has_typedef_foobar<foo>::value << std::endl;

}

</syntaxhighlight>

</source>

With the standardisation of the detection idiom in the [http://en.cppreference.com/w/cpp/experimental/lib_extensions_2 Library fundamental v2 (n4562)] proposal, the above code could be re-written as follows:

<sourcesyntaxhighlight lang="cpp">

#include <iostream>

#include <type_traits>

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std::cout << std::is_detected<has_typedef_foobar_t, foo>::value << std::endl;

}

</sourcesyntaxhighlight>

The developers of [[Boost C++ Libraries|Boost]] used SFINAE in boost::enable_if<ref name="enable_if">[http://www.boost.org/doc/libs/release/libs/utility/enable_if.html Boost Enable If]</ref> and in other ways.