How to overcome the square bracket issue with lambda expression C++

Asked Dec 12 '21 at 07:01

Active Dec 12 '21 at 07:18

Viewed 30 times

Here's a Function implementation from Bjarne's book

typedef double Fct(double);

Function::Function(Fct f, double r1, double r2, Point xy, int count, double xscale, double yscale)

and here's the code creating Function e

Function e{[n](double x) {return expcalc(x,n);},rmin,rmax,orig,200,xscale,yscale };

The line above gives the following error when compiling:

No instance of constructor Function::Function matches the argument list.

Note 1: this function is used to calculate exp(x) using Taylor series

Note 2: the code compiles if I remove n from lambda's square brackets

Note 3: Bjarne's used the exact same implementation in his book, not sure why it's not working here

Note 4: I'm using MVS19 community version

edited Dec 12 '21 at 07:18

Nicol Bolas

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asked Dec 12 '21 at 07:01

zizou

@S.M. Yes it can, but the syntax is a bit obscure : auto lambda = [](double x) {return 2.0 * x; }; Fct fun = +lambda; – Pepijn Kramer Dec 12 '21 at 07:14
I need to keep n – zizou Dec 12 '21 at 07:16
Bjarne used THIS EXACT implementation and it works in his case – zizou Dec 12 '21 at 07:16
@zizou: It is a duplicate. Lambdas can be converted to a function pointer, but only if they don't capture anything. – Nicol Bolas Dec 12 '21 at 07:18
@PepijnKramer: It's a compile error; "life cycle" is irrelevant. – Nicol Bolas Dec 12 '21 at 07:19
this is the whole code for (int n = 0 ; n<50 ; ++n){ Function e{[n](double x) {return expcalc(x,n);},rmin,rmax,orig,200,xscale,yscale }; } – zizou Dec 12 '21 at 07:20
@NicolBolas I missed that bit, you're right. Then just use std::function instead of a function pointer. – Pepijn Kramer Dec 12 '21 at 07:21
I copied the whole thing from Bjarne's book, why it's not working if he used it ? – zizou Dec 12 '21 at 07:22
1

Bjarne can also make mistakes ;) – Pepijn Kramer Dec 12 '21 at 07:23
Oh and (implementations of) compilers too – Pepijn Kramer Dec 12 '21 at 07:24
@zizou: Books can have typos. The language is what it is regardless. – Nicol Bolas Dec 12 '21 at 07:26
the point is this Function is not the std::function, another point is this is not about pointers. Off course everyone makes mistakes, but the idea here this implementation of lambda in this is inside a loop, so it is taking the value n. So here there's no room for suspicion about typos or whatever. – zizou Dec 12 '21 at 07:29

How to overcome the square bracket issue with lambda expression C++

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