Can C 0x Lambda Captures Be Modified Without Being Const?

Can C 0x Lambda Captures Be Modified without Being Const?

Lambda captures in C 0x typically capture variables by reference, which are inherently constant. However, certain scenarios require the modification of captured variables, raising the question, "Can captured values be made non-const?"

Consider a library functor, foo, with a non-const method, operator(), that needs to be called within a lambda. The code snippet below attempts this but fails to compile:

<code class="c++">struct foo
{
  bool operator () ( const bool &amp; a )
  {
    return a;
  }
};

auto bar = [=] () -> bool
{
  afoo(true);
};</code>

The compilation error arises because the lambda's operator() is implicitly marked as const due to the captured values being constant. To resolve this issue, C 0x provides the mutable keyword.

<code class="c++">auto bar = [=] () mutable -> bool
{
  afoo(true);
};</code>

By adding mutable, the lambda's operator() is no longer const, allowing the captured variables to be modified within the lambda. This enables the modification of library functors or other non-const captured values as needed within lambdas.

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