Databases
StackOverflow2013
Postsdbo
POreturning a std::function wrapped lambda that invokes a specified pointer to member function
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POreturning a std::function wrapped lambda that invokes a specified pointer to member function
    primarykey
    data
    text
    <p>I have a case where I can pass a lambda to std::sort, and I can also supply the predicate by calling a function that returns a std::function which wraps this same lambda, but, if I try to call a similar function which allows me to specify a pointer to member function, this compiles but fails at run time.</p> <p>This works:</p> <pre><code>std::sort(myContainer.begin(), myContainer.end(), [&amp;](type lhs, type rhs) { return MyMemberFunction(lhs, rhs); }); </code></pre> <p>And this works:</p> <pre><code>std::function&lt;bool(type,type)&gt; ReturnPred() { std::function&lt;bool(type,type)&gt; pred = [&amp;](type lhs, type rhs) { return MyMemberFunction(lhs, rhs); }; return pred; } std::sort(myContainer.begin(), myContainer.end(), ReturnPred()); </code></pre> <p>But this does not work:</p> <pre><code>std::function&lt;bool(type,type)&gt; ReturnGeneralPred( bool(MyClass::Func*)(type lhs, type rhs)) { std::function&lt;bool(type,type)&gt; pred = [&amp;](type lhs, type rhs) { return (this-&gt;*Func)(lhs, rhs); }; return pred; } std::function&lt;bool(type,type)&gt; ReturnThisPred() { return ReturnGeneralPred(&amp;MyClass::MyMemberFunction); } std::sort(myContainer.begin(), myContainer.end(), ReturnThisPred()); </code></pre> <p>When I try to do it this last generic way and I step through the debugger, when std::sort calls the predicate, it steps into what I've call ReturnGeneralPred above, and Func seems to be undefined, as though it was a local variable that went out of scope.</p> <p>At the moment I can get the same functionality by losing some genericness, but I was wondering if maybe there's a way to accomplish what I was trying to do.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. USCharles J. Daniels
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. COTry to capture local `Func` by value instead of by reference.
      singulars
      1. POreturning a std::function wrapped lambda that invokes a specified pointer to member function
      1. USJarod42
    2. CO"Func seems to be undefined, as though it was a local variable that went out of scope.". I'm almost tempted to close this as "Missing an actual question.", given that the only apparent question is already answered.
      singulars
      1. POreturning a std::function wrapped lambda that invokes a specified pointer to member function
      1. USMSalters
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload