Compiler Error C3491

'var': a by-value capture cannot be modified in a non-mutable lambda

A non-mutable lambda expression cannot modify the value of a variable that is captured by value.

To correct this error

• Declare your lambda expression with the mutable keyword, or

• Pass the variable by reference to the capture list of the lambda expression.

Examples

The following example generates C3491 because the body of a non-mutable lambda expression modifies the capture variable m:

// C3491a.cpp

int main()
{
   int m = 55;
   [m](int n) { m = n; }(99); // C3491
}

The following example resolves C3491 by declaring the lambda expression with the mutable keyword:

// C3491b.cpp

int main()
{
   int m = 55;
   [m](int n) mutable { m = n; }(99);
}

See also

Lambda Expressions