Passing multiple arguments to a threaded function

Question

I have a function called workForThread, that takes two arguments, and returns void. I would like to thread this function using something like:

thread(workForThread,a,b);

Where a and b are of the appropriate types. The above code does not compile, giving a "too many arguments for call" error ("error C2197: 'void (__cdecl *)(char *)' : too many arguments for call")

How can I resolve this?

Note: I have looked at these two questions, but the resolutions that work there do not seem to work for me. Additionally, I have a feeling there is a way to do it built into c++11, and that is what I am looking for.

C++11 thread has a (templated) constructor that does exactly that. — Mat, May 07 '12 at 18:18
What version of VS do you have? Maybe it doesn't support that. — Mat, May 07 '12 at 18:24
Not familiar with VS, but I think that version should work, and I think that error message looks very strange. What's the signature of the function you're using? — Mat, May 07 '12 at 18:26
Just realized there was a shadowing definition it now works. — soandos, May 07 '12 at 18:33
Voting to close, as the real problem was in code the OP didn't show us. — ildjarn, May 07 '12 at 18:48

score 13 · Answer 1 · answered May 07 '12 at 18:31

13

In C++11, the way to do it is more or less the way you have attempted:

std::thread myThread(workForThread,a,b);

provided workForThread is a (non-member) function that takes those two parameters.

answered May 07 '12 at 18:31

juanchopanza

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maybe add that the function may not be overloaded. I think that is the problem in his case. – Stephan Dollberg May 07 '12 at 18:34
@bamboon what do you mean not overloaded? – juanchopanza May 07 '12 at 18:36
let's say you have workforthread(int, int) and workforthread(int) then this won't compile. – Stephan Dollberg May 07 '12 at 18:38
@bamboon neither will any other code that calls the function with two arguments. OP said "I have a function called workForThread, that takes two arguments, and returns void.". – juanchopanza May 07 '12 at 18:39

score 7 · Accepted Answer · edited May 23 '17 at 12:03

7

When using C++11 you could use a lambda function which may use (non-formal) parameter of the context. "Capturing"

Something like

void doIt (int a, int b) {  // do something, your workForThread
}

..
int a = 1;
int b = 2;

std:thread r ([=](){doIt (a, b); return 1;});

When only calling a single function juanchopanza answer may be somewhat more efficient since no new function will be created.

The lambda version allows you to configure more. Let say you are starting threads which calls in the end 2 functions out of 3. juanchopanza approach would require NAMED functions for each permutation.

At the moment I consider the difference of both approaches to be mostly a matter of taste.

When you like to read more about lambda functions

What is a lambda expression in C++11?

edited May 23 '17 at 12:03

Community

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answered May 07 '12 at 18:19

stefan bachert

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Can you please show me what you mean? I do not have experience with c++ lambdas – soandos May 07 '12 at 18:20
This now compiles, but casues a fatal error in "__declspec(noinline) void __cdecl _CRT_DEBUGGER_HOOK(int _Reserved)" any idea what is causing this? – soandos May 07 '12 at 18:33
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Absolutely no need for the lambda function in C++11. – juanchopanza May 07 '12 at 18:34
Probably the lambda need a dummy return value – stefan bachert May 07 '12 at 18:34
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@stefanbachert it is irrelevant, you really don't need the lambda here. It is entirely pointless. – juanchopanza May 07 '12 at 18:35

Passing multiple arguments to a threaded function

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