Std bind equivalent to lambda for binding a member function to a std function

Question

I can bind a private member function using a lambda. I'm struggling to write the equivalent using std::bind. This is my attempt, but it doesn't compile.

#include <functional>

class A { 
    private: 
        double foo(double x, double y); 
    public:          
        A(); 
        std::function<double(double,double)> std_function;      
 }; 

A::A() {
    // This works:
    //std_function = [this](double x, double y){return foo(x,y);};

    std_function = std::bind(&A::foo,this,std::placeholders::_1));
}

songyuanyao · Accepted Answer · 2018-02-23 11:02:13Z

2

std_function is supposed to take 2 parameters, but you're only specifying one. Note that the placeholders are used for arguments to be bound when std_function is invoked later.

Change it to

std_function = std::bind(&A::foo, this, std::placeholders::_1, std::placeholders::_2);

edited Feb 23, 2018 at 11:02

answered Feb 23, 2018 at 10:45

songyuanyao

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6 Comments

Blasco Over a year ago

ahhh, I got it wrong I thought the placeholder meant to bind this to the first argument. Ok, great! thank you very much!

Blasco Over a year ago

I feel bad I spent so much time in such a simple mistake.

songyuanyao Over a year ago

@WooWapDaBug It's not bad if it helps you clarify the usage about placeholders.

Passer By Over a year ago

Note using lambdas is almost always just a better choice

Blasco Over a year ago

@PasserBy I think the same way, I was actually explaining this to someone and I wanted to show this person how uglier it looked with bind, but I couldn't make it work with bind because I always use a lambda. It was for the sake of an example

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