How to Handle Lambda Capture Errors When Capturing External Variables in C ?

Lambda Capture Error: Capturing External Variables

When invoking std::remove_if with a lambda function, it's essential to specify the variables that need to be captured from the enclosing scope. In this context, the provided code attempts to filter the m_FinalFlagsVec based on an external variable flagId, but encounters the error:

"error C3493: 'flagId' cannot be implicitly captured because no default capture mode has been specified"

Solution: Explicit Capture

To resolve this issue, we must explicitly declare the capture of flagId within the lambda expression using square brackets, followed by the capture mode (by value, by reference, or by const value). For example, capturing flagId by reference:

<code class="cpp">auto new_end = std::remove_if(m_FinalFlagsVec.begin(), m_FinalFlagsVec.end(),
        [&flagId](Flag& device)
    { return device.getId() == flagId; });</code>

This syntax explicitly captures flagId as a reference, allowing the lambda expression to access and manipulate its value.

Capture Modes

The following table summarizes the different capture modes available in C lambda expressions:

Capture Mode	Syntax	Behavior
Capture by value	[flagId]	Creates a copy of flagId inside the lambda
Capture by reference	[&flagId]	Captures a reference to flagId
Capture by const value	[flagId] (const-qualified)	Captures a constant copy of flagId
Capture by mutable value	[flagId] (mutable qualifier)	Captures a value of flagId that can be modified inside the lambda
Capture by const reference	[&flagId] = std::as_const(flagId) (C 17 )	Captures a constant reference to flagId

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