~ubuntu-branches/debian/sid/boost1.49/sid

<a name="boost_typetraits.reference.has_bit_or_assign"></a><a class="link" href="has_bit_or_assign.html" title="has_bit_or_assign">has_bit_or_assign</a>

</h3></div></div></div>

<pre class="programlisting">template <class Lhs, class Rhs=Lhs, class Ret=dont_care>

struct has_bit_or_assign : public <code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code> {};

</pre>

Inherits: If (i) <code class="computeroutput">lhs</code>

of type <code class="computeroutput">Lhs</code> and <code class="computeroutput">rhs</code> of type <code class="computeroutput">Rhs</code>

can be used in expression <code class="computeroutput">lhs|=rhs</code>,

and (ii) <code class="computeroutput">Ret=dont_care</code> or the result of expression

<code class="computeroutput">lhs|=rhs</code> is convertible to <code class="computeroutput">Ret</code>

then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,

otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.

The default behaviour (<code class="computeroutput">Ret=dont_care</code>)

is to not check for the return value of binary <code class="computeroutput">operator|=</code>. If <code class="computeroutput">Ret</code>

is different from the default <code class="computeroutput">dont_care</code>

type, the return value is checked to be convertible to <code class="computeroutput">Ret</code>.

Convertible to <code class="computeroutput">Ret</code> means

that the return value of the operator can be used as argument to a function

expecting <code class="computeroutput">Ret</code>:

<pre class="programlisting">void f(Ret);

Lhs lhs;

Rhs rhs;

f(lhs|=rhs); // is valid if has_bit_or_assign<Lhs, Rhs, Ret>::value==true

</pre>

If <code class="computeroutput">Ret=void</code>, the return type is checked to be exactly

<code class="computeroutput">void</code>.

Header: <code class="computeroutput">#include

or <code class="computeroutput">#include <boost/type_traits/has_operator.hpp></code>

or <code class="computeroutput">#include <boost/type_traits.hpp></code>

Examples:

<code class="computeroutput">has_bit_or_assign<Lhs, Rhs, Ret>::value_type</code> is the type <code class="computeroutput">bool</code>.

</blockquote></div>

<code class="computeroutput">has_bit_or_assign<Lhs, Rhs, Ret>::value</code> is a <code class="computeroutput">bool</code>

integral constant expression.

</blockquote></div>

<code class="computeroutput">has_bit_or_assign<int>::value</code> is a <code class="computeroutput">bool</code>

integral constant expression that evaluates to <code class="computeroutput">true</code>.

</blockquote></div>

<code class="computeroutput">has_bit_or_assign<long></code>

inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.

</blockquote></div>

inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.

</blockquote></div>

<code class="computeroutput">has_bit_or_assign<const int, int></code>

inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.

</blockquote></div>

inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.

</blockquote></div>

<code class="computeroutput">has_bit_or_assign<int, int, std::string></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.

</blockquote></div>

See also: <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator

100

Type Traits</a>

101

102

103

Limitation:

104

105

106

Requires a compiler with working SFINAE.

107

</li></ul></div>

108

109

Known issues:

110

111

112

113

This trait cannot detect whether binary <code class="computeroutput">operator|=</code> is public or not: if <code class="computeroutput">operator|=</code>

114

is defined as a private member of <code class="computeroutput">Lhs</code>

115

then instantiating <code class="computeroutput">has_bit_or_assign<Lhs></code> will produce a compiler error. For

116

this reason <code class="computeroutput">has_bit_or_assign</code>

117

cannot be used to determine whether a type has a public <code class="computeroutput">operator|=</code>

118

or not.

119

<pre class="programlisting">struct A { private: void operator|=(const A&); };

120

boost::has_bit_or_assign<A>::value; // error: A::operator|=(const A&) is private

121

</pre>

122

</li>

123

124

There is an issue if the operator exists only for type <code class="computeroutput">A</code> and <code class="computeroutput">B</code>

125

is convertible to <code class="computeroutput">A</code>.

126

In this case, the compiler will report an ambiguous overload.

127

<pre class="programlisting">struct A { };

128

void operator|=(const A&, const A&);

129

struct B { operator A(); };

130

131

boost::has_bit_or_assign<B>::value; // error: ambiguous overload

132

</pre>

133

</li>

134

135

There is an issue when applying this trait to template classes. If <code class="computeroutput">operator|=</code>

136

is defined but does not bind for a given template type, it is still detected

137

by the trait which returns <code class="computeroutput">true</code>

138

instead of <code class="computeroutput">false</code>. Example:

139

140

<pre class="programlisting">#include <boost/type_traits/has_bit_or_assign.hpp>

141

#include <iostream>

142

143

template <class T>

144

struct contains { T data; };

145

146

template <class T>

147

bool operator|=(const contains<T> &lhs, const contains<T> &rhs) {

148

return f(lhs.data, rhs.data);

149

}

150

151

class bad { };

152

class good { };

153

bool f(const good&, const good&) { }

154

155

int main() {

156

157

// works fine for contains<good>

158

std::cout<<boost::has_bit_or_assign< contains< good > >::value<<'\n'; // true

159

contains<good> g;

160

g|=g; // ok

161

// does not work for contains<bad>

162

std::cout<<boost::has_bit_or_assign< contains< bad > >::value<<'\n'; // true, should be false

163

contains<bad> b;

164

b|=b; // compile time error

165

return 0;

166

}

167

</pre>

168

</li>

169

170

<code class="computeroutput">volatile</code> qualifier is not

171

properly handled and would lead to undefined behavior

172

</li>

173

</ul></div>

174

</div>

175

176

177

178

Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,

179

Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten

180

Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven

181

Watanabe

182

Distributed under the Boost Software License, Version 1.0. (See accompanying

183

file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)

184

185

</div></td>

186

</tr></table>

187

<hr>

188

189

190

</div>

191

</body>

192

</html>

Older »