std::remove_if with lambda predicate

Question

I would like to remove an object from a vector based on a lambda predicate:

class tmr { public: tmr(); ~tmr(); static void start(); bool is_time_out(double sec); double t_elapsed(); }; struct sobj { int count; bool tflag; int ID; tmr timer; friend bool is_stime(timer& objtimer,double sec) { return objtimer.is_time_out(sec); } };

somewhere in the main program, I populate a vector<sobj>, then after some time, I want to remove the element whose ID is specified and whose timer has elapsed.

I did this , and it complains about not being able to convert void to bool

sobj strobj; vector<sobj> vecobj; vecobj.erase(std::remove_if(vecobj.begin(),vecobj.end(),[&](const sobj& mysobj){return ( mysobj.ID== THE_ID && mysobj.is_stime(mysobj.timer,5));}),vecobj.end());

Could you show your compiler, its version, and paste the actual error? — Useless
– Useless, Commented Apr 2, 2015 at 15:36
@Useless: The unhelpful part is that Sam didn't include this part of the error: foo.cc:11:16: error: use of undeclared identifier 'is_sobj' — Bill Lynch
– Bill Lynch, Commented Apr 2, 2015 at 15:49
Can you post the actual error message, instead of your interpretation of it? highlight, copy, paste into text editor, add 4 spaces at front of each line (s/^/ / assuming your editor is vi), and edit it into your post. — Yakk - Adam Nevraumont
– Yakk - Adam Nevraumont, Commented Apr 2, 2015 at 16:03
first, the compiler is C++11. I did what's recommended in the answer , the error is : error: passing ‘const timer’ as ‘this’ argument of ‘bool timer::is_time_out(double)’ discards qualifiers [-fpermissive] return to.obj_timer.is_time_out(sec); — Sam Gomari
– Sam Gomari, Commented Apr 2, 2015 at 16:04
ok, i'll post the output, i just need to edit it, am unable to disclose some output information — Sam Gomari
– Sam Gomari, Commented Apr 2, 2015 at 16:08

Community · Accepted Answer · 2017-05-23 12:25:18Z

First things first:

Let's note that this has very little to do with the lambda. The following code will also fail to compile:

sobj strobj; is_stime(strobj.timer, 5);

Steps taken:

Let's reduce your test case down..
is_stime() needs to take a const reference, or your lambda needs to pass a non-const reference.
is_stime() is not visible to your lambda. Would you like to know more?

Reduced Code:

#include <iostream> #include <vector> using namespace std; int THE_ID; class tmr { }; struct sobj { int ID; tmr timer; friend bool is_stime(tmr const & objtimer, double sec); }; bool is_stime(tmr const & objtimer, double sec) { return true; } int main() { vector<sobj> vecobj; vecobj.erase(std::remove_if(vecobj.begin(),vecobj.end(),[&](const sobj& mysobj){return ( mysobj.ID == THE_ID && is_stime(mysobj.timer,5));}),vecobj.end()); }

Is it possible to render "is_time_out(double sec)" of tmr class accessible/visible to lambda function, i can bypass struct's friend function
you can see that tmr class has a function "is_time_out" , and since tmr object is a member of the sobj struct , i was wondering if one can simply have the lambda return the result of the "is_time_out" , without sobj having to have a function. Just directly access member function of tmr and return its result from lambda function to remove it from vector.
@SamGomari: I don't see any reason why you couldn't do that.
The issue was the const in [&](const sobj& mysobj){return ... } . Thanks a lot

Useless · Accepted Answer · 2015-04-02 15:20:47Z

1

your lambda is missing a return type:

[&](const sobj& mysobj)->bool

edited Apr 2, 2015 at 15:20

Useless

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answered Apr 2, 2015 at 15:20

shoosh

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3 Comments

Bill Lynch Over a year ago

Shouldn't this take care of it: if the body consists of nothing but a single return statement with an expression, the return type is the type of the returned expression (after rvalue-to-lvalue, array-to-pointer, or function-to-pointer implicit conversion)

vsoftco Over a year ago

not sure the missing return is the issue, seems that even the single-return statement was relaxed, see e.g stackoverflow.com/questions/28955478/…

Sam Gomari Over a year ago

I do not think the return type is the issue.

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