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copied!<pre><code>temp temp1; </code></pre> <p>This calls <code>temp</code>'s default constructor on the instance called <code>temp1</code>.</p> <pre><code>temp temp2 = temp(); </code></pre> <p>This calls <code>temp</code>'s default constructor on a temporary object, then calls the compiler-generated copy-constructor on <code>temp2</code> with the temporary object as the argument (this of course assumes that the compiler doesn't elide copies; it depends on your compiler's optimization settings).</p> <p>As for why you get different initialized values, section 8.5 of the standard is relevant:</p> <hr> <h2>8.5 Initializers [dcl.init]</h2> <p><h3>Paragraph 5:</h3> To <em>zero-initialize</em> an object of type <code>T</code> means:</p> <ul> <li>if <code>T</code> is a scalar type (3.9), the object is set to the value of 0 (zero) converted to <code>T</code>;</li> <li>if <code>T</code> is a non-union class type, each nonstatic data member and each base-class subobject is zero-initialized;</li> <li>if <code>T</code> is a union type, the object’s first named data member is zero-initialized;</li> <li>if <code>T</code> is an array type, each element is zero-initialized;</li> <li>if <code>T</code> is a reference type, no initialization is performed.</li> </ul> <p>To <em>default-initialize</em> an object of type <code>T</code> means:</p> <ul> <li>if <code>T</code> is a non-POD class type (clause 9), the default constructor for <code>T</code> is called (and the initialization is ill-formed if <code>T</code> has no accessible default constructor);</li> <li>if <code>T</code> is an array type, each element is default-initialized;</li> <li>otherwise, the object is zero-initialized.</li> </ul> <p>To value-initialize an object of type <code>T</code> means:</p> <ul> <li>if <code>T</code> is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for <code>T</code> is called (and the initialization is ill-formed if <code>T</code> has no accessible default constructor);</li> <li>if <code>T</code> is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;</li> <li>if <code>T</code> is an array type, then each element is value-initialized;</li> <li>otherwise, the object is zero-initialized.</li> </ul> <p><h3>Paragraph 7:</h3> An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.</p> <p><h3>Paragraph 9:</h3> If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor. Otherwise, if no initializer is specified for a nonstatic object, the object and its subobjects, if any, have an indeterminate initial value; if the object or any of its subobjects are of const-qualified type, the program is ill-formed.</p> <h2>12 Special Member Functions [special]</h2> <p><h3>Paragraph 7:</h3> An implicitly-declared default constructor for a class is implicitly defined when it is used to create an object of its class type (1.8). The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with an empty mem-initializer-list (12.6.2) and an empty function body.</p> <h2>12.6.2 Initializing bases and members [class.base.init]</h2> <p><h3>Paragraph 4:</h3> If a given nonstatic data member or base class is not named by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then</p> <ul> <li>If the entity is a nonstatic data member of (possibly cv-qualified) class type (or array thereof) or a base class, and the entity class is a non-POD class, the entity is default-initialized (8.5). If the entity is a nonstatic data member of a const-qualified type, the entity class shall have a user-declared default constructor.</li> <li>Otherwise, the entity is not initialized. If the entity is of const-qualified type or reference type, or of a (possibly cv-qualified) POD class type (or array thereof) containing (directly or indirectly) a member of a const-qualified type, the program is ill-formed.</li> </ul> <hr> <p>So now that the rules have been laid out, let's see how they apply:</p> <pre><code>temp temp1; </code></pre> <p><code>temp</code> is a non-POD type (because it has a <code>std::string</code> member), and since no initializer is specified for <code>temp1</code>, it will be default-initialized (8.5/9). This calls the default constructor (8.5/5). <code>temp</code> has an implicit default constructor (12/7) which default-initializes the <code>std::string</code> member and the <code>int</code> member isn't initialized at all (12.6.2/4).</p> <pre><code>temp temp2 = temp(); </code></pre> <p>On the other hand, the temporary <code>temp</code> object is value-initialized (8.5/7), which value-initializes all data members (8.5/5), which calls the default constructor in the <code>std::string</code> member and zero-initializes the <code>int</code> member (8.5/5).</p> <p>Of course, if you much rather not have to refer to the standard in 5+ different places, just ensure that you explicitly initialize everything (e.g. <code>int i = 0;</code> or using initializer lists).</p>

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