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    <p>Yes, the code is standard conforming. The <code>+</code> triggers a conversion to a plain old function pointer for the lambda.</p> <p>What happens is this:</p> <p>The compiler sees the first lambda (<code>[]{}</code>) and generates a closure object according to §5.1.2. As the lambda is a <strong>non-capturing</strong> lambda, the following applies:</p> <blockquote> <h3>5.1.2 Lambda expressions [expr.prim.lambda]</h3> <p><sup>6</sup> The closure type for a <em>lambda-expression</em> with no <em>lambda-capture</em> has a public non-virtual non-explicit const conversion function to pointer to function having the same parameter and return types as the closure type’s function call operator. The value returned by this conversion function shall be the address of a function that, when invoked, has the same effect as invoking the closure type’s function call operator.</p> </blockquote> <p>This is important as the unary operator <code>+</code> has a set of built-in overloads, specifically this one:</p> <blockquote> <h3>13.6 Built-in operators [over.built]</h3> <p><sup>8</sup> For every type <code>T</code> there exist candidate operator functions of the form</p> <p>&nbsp;&nbsp;&nbsp;&nbsp;<code>T* operator+(T*);</code></p> </blockquote> <p>And with this, it's quite clear what happens: When operator <code>+</code> is applied to the closure object, the set of overloaded built-in candidates contains a conversion-to-any-pointer and the closure type contains exactly one candidate: The conversion to the function pointer of the lambda.</p> <p>The type of <code>test</code> in <code>auto test = +[]{};</code> is therefore deduced to <code>void(*)()</code>. Now the second line is easy: For the second lambda/closure object, an assignment to the function pointer triggers the same conversion as in the first line. Even though the second lambda has a different closure type, the resulting function pointer is, of course, compatible and can be assigned.</p>
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    1. POA positive lambda: '+[]{}' - What sorcery is this?
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    1. USDaniel Frey
    1. USDaniel Frey
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    1. POA positive lambda: '+[]{}' - What sorcery is this?
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    1. COFascinating. And what's the point of unary `+` for pointers? I understand it exists for numeric types for completeness with unary `-`. But unary `-` with pointers makes no sense.
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      1. USTadeusz Kopec
    2. CO@TadeuszKopec That's a good question and I'm afraid I don't have an answer for that. Note that for pointers 13.6/8 only defines `+`, not `-`.
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      1. USDaniel Frey
    3. COIt is useful if you want to force decay of arrays or functions and if you want to force promotion of small integer types or unscoped enumerations.
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      1. USJohannes Schaub - litb
 

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