Lambda function: Replacement for global functions or function objects(functors), can generate unnamed function objects.
- General syntax:
[capture variables] (input arguments) <mutable> -> return type { function implementation } -
Return type deduction
When lambda is only one return expression, return type deduced automatically.
-
Capture-default
Syntax Description [=var] passes var by value [&var] passes var by reference [=] passes all variables by value [&] passes all variables by reference [=, &var] passes all variables by value except var which is passed by reference -
Keyword mutable
Variables captured by value, can’t be changed because the lambda function is const in the generated function object. To make the lambda function non-const, define it as mutable. The original variable stays unchanged, but within the function body, the captured variable can be modified.
-
Functor Rule
A functor’s return value should only depend on the value passed as argument and not impacted by “stage variables”.** The stage variable can be captured values, global static variables defined outside, etc.
-
Capture-default in class
Syntax Description [this] captures only the this pointer [=] captures this pointer + local variable by value [&] captures this pointer + local variable by reference
Inside lambda function can access member by: this->member or member.
Sample code:
// Class to calculate the sine value of a vector with input values.
class CalculateSine {
private:
double m_amplitude;
public:
CalculateSine(double amplitude): m_amplitude(amplitude) {}
// Calculate sine.
vector<double> Calculate(vector<double>& input) {
vector<double> result(input.size()); // Create result vector.
transform(input.begin(), input.end(), result.begin(),
[this](double v) -> double
{ return m_amplitude*sin(6.28319*v/360.0); } );
return result; // Return the result. }
};
Transform the input vector using a lambda function. The lambda function must capture the m_amplitude member variable. [m_amplitude] or [this->m_amplitude] does not work. Use [this] instead. Using [=] or [&] will also capture the this pointer. Now you can use the members implicitly(m_amplitude) or explicitly (this->m_amplitude).
