How a local variable can be taken in this C++ lambda expression? - c++

Question

In the following code:
#include "Simple_window.h"
#include "Graph.h"
int fac(int n) // factorial(n); n!
{
int r = 1;
while(n>1) {
r *= n;
--n;
}
return r;
}
double term(double x,int n) { return pow(x,n)/fac(n); }
double expe(double x,int n) // sum of n terms for x
{
double sum = 0;
for(int i = 0; i<n; ++i) sum += term(x,i);
return sum;
}
int main() {
Simple_window win {Point{100,100},xmax,ymax,""};
for(int n = 0; n<50; ++n) {
ostringstream ss;
ss << "exp approximation; n==" << n;
win.set_label(ss.str());
// get next approximation:
Function e {[n](double x) { return expe(x,n); },
-10,10,Point{300,300},200,30,30; // ***this line doesn't compile***
win.attach(e);
win.wait_for_button();
win.detach(e);
}
}
From the book "Principles and Practice using C++" by Stroustrup, the local variable n isn't taken when I try to compile it, and gives the error message:
No instance of the constructor Graph_lib::Function::Function coincides with the argument list
What is the problem?
By the way, the support code used for the books is https://web.archive.org/web/20191217104940/http://www.stroustrup.com/Programming/PPP2code

Answer 1

Your post is not close to a Minimal Reproducible Example
Here is an example for Minimal Reproducible Example.
In Graph.h Function takes a variable of Fct.
Where Fct is typedef double Fct(double);.
According to this post, lambda expression doesn't translate to function automatically, unless it doesn't create closure object by capturing nothing.
Check this example
typedef double Fct ( double );
typedef double bct ( double, int );
struct Foo{
Foo( Fct f ){};
};
struct bar{
bar( bct f){};
};
int main(){
int n(1);
Foo f{ [](double x){ return x; } };
//Foo f{ [n](double x){ return x*n; } }; // <=== this won't compile
bar b{[](double x, int n){return x*n;}};
}
To pass the n into function f without closure, you may
Change signature from typedef double Fct ( double ); to typedef double Fct ( double, int ); Like my example of bar
Write a function with a constant n.
(Strongly not suggested, unless you never maintain the code ) Global variable to make it possible to change n outside the function.