Substitution failure is not an error: Difference between revisions

Revision as of 13:22, 22 September 2015

Revision as of 03:49, 8 October 2015

Line 45:

static no& test(...);

// If the "sizeof" of the result of calling test<T>(0nullptr) would be equal to sizeof(yes),

// the first overload worked and T has a nested type named foobar.

static const bool value = sizeof(test<T>(nullptr)) == sizeof(yes);

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</source>

When <code>T</code> has the nested type <code>foobar</code> defined, the instantiation of the first <code>test</code> works and 0 is successfully passed as 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.)

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.