Substitution failure is not an error: Difference between revisions

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{{Short description|C++ programming technique}}

{{useUse dmy dates|date=JanuaryDecember 20122023}}

'''Substitution failure is not an error''' ('''SFINAE''') is a principle in [[C++]] where an invalid substitution of [[Template (C++)|template]] parameters is not in itself an error. David Vandevoorde first introduced the acronym SFINAE to describe related programming techniques.<ref>{{cite book | last=Vandevoorde | first=David |author2=Nicolai M. Josuttis | title=C++ Templates: The Complete Guide | publisher=Addison-Wesley Professional | year=2002 | isbn=0-201-73484-2}}</ref>

Specifically, when creating a candidate set for [[overload resolution]], some (or all) candidates of that set may be the result of instantiated templates with (potentially deduced) template arguments substituted for the corresponding template parameters. If an error occurs during the substitution of a set of arguments for any given template, the compiler removes the potential overload from the candidate set instead of stopping with a compilation error, provided the substitution error is onethat the C++ standard grantspermits discarding such treatmenta substitution error as mentioned.<ref>International Organization for Standardization. "ISO/IEC 14882:2003, Programming languages &mdash;– C++", § 14.8.2.</ref> If one or more candidates remain and overload resolution succeeds, the invocation is well-formed.

==Example==

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f<int>(10); // Call #2. Without error (even though there is no int::foo)

// thanks to SFINAE.

return 0;

}

</syntaxhighlight>

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template <typename T>

struct has_typedef_foobarHasTypedefFoobar {

// Types "yes" and "no" are guaranteed to have different sizes,

// specifically sizeof(yes) == 1 and sizeof(no) == 2.

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};

struct fooFoo {

typedef float foobar;

};

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int main() {

std::cout << std::boolalpha;

std::cout << has_typedef_foobarHasTypedefFoobar<int>::value << std::endl; // Prints false

std::cout << has_typedef_foobarHasTypedefFoobar<fooFoo>::value << std::endl; // Prints true

return 0;

}

</syntaxhighlight>

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#include <iostream>

#include <type_traits>

template <typename... Ts>

using void_t = void;

template <typename T, typename = void>

struct has_typedef_foobarHasTypedefFoobar : std::false_type {};

template <typename T>

struct has_typedef_foobarHasTypedefFoobar<T, std::void_t<typename T::foobar>> : std::true_type {};

struct fooFoo {

using foobar = float;

};

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int main() {

std::cout << std::boolalpha;

std::cout << has_typedef_foobarHasTypedefFoobar<int>::value << std::endl;

std::cout << has_typedef_foobarHasTypedefFoobar<fooFoo>::value << std::endl;

return 0;

}

</syntaxhighlight>

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template <typename T>

using has_typedef_foobar_tHasTypedefFoobarUnderlying = typename T::foobar;

struct fooFoo {

using foobar = float;

};

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int main() {

std::cout << std::boolalpha;

std::cout << std::is_detected<has_typedef_foobar_tHasTypedefFoobarUnderlying, int>::value << std::endl;

std::cout << std::is_detected<has_typedef_foobar_tHasTypedefFoobarUnderlying, fooFoo>::value << std::endl;

return 0;

}

</syntaxhighlight>

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.

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{{C++ programming language}}

{{use dmy dates|date=January 2012}}

[[Category:C++]]

[[Category:Articles with example C++ code]]