C++ sort using lambda errors

Question

I'm trying to sort an array using a lambda to use another array in the comparison function. Inside a larger function, I have this code:

std::sort(arr.begin(), arr.end(),[] (int& a, int& b)
{       
    return (*i)[a] < (*i)[b];
});

I get the following errors in the containing function:

mcmc.cpp:139: error: expected primary-expression before ‘[’ token
mcmc.cpp:139: error: expected primary-expression before ‘]’ token
mcmc.cpp:139: error: expected primary-expression before ‘int’
mcmc.cpp:139: error: expected primary-expression before ‘int’

When compiling, I'm including the -std=c++0x option as well.

I'm confused about what's going on. For some reason, it doesn't seem to recognize my syntax as valid. I did a yum update just in case, but it still seems as if it just doesn't recognize the use of lambdas.

From your I'm including the -std=c++0x option comment, I gather you're using G++, so the question is which version of G++ exactly? Not every one supports lambdas. See gcc.gnu.org/projects/cxx0x.html (this page contains the information about C++11 compatibility per G++ version, I consult it regularly but somehow the site seems to be down right now -- try again later) — syam
– syam, Commented Mar 19, 2013 at 1:06
It looks like you're missing a paren after the closure argument. — bchurchill
– bchurchill, Commented Mar 19, 2013 at 1:08
@syam I have v 4.4.7 gcc installed. I thought g++ was just the cpp command to run gcc. I just did a yum update, so I should have the latest gcc. — saccharine
– saccharine, Commented Mar 19, 2013 at 1:10
nope, I made a mistake. There's no syntax error. It almost compiles find with my g++ version 4.6.3 -- although you need to capture i in the closure (e.g. put it in the square brackets). — bchurchill
– bchurchill, Commented Mar 19, 2013 at 1:39

syam · Accepted Answer · 2013-03-19 01:13:07Z

8

C++11 lambdas require GCC/G++ 4.5 at least, they won't work with G++ 4.4.

See http://gcc.gnu.org/projects/cxx0x.html (or, since it is down right now, the cached version).

answered Mar 19, 2013 at 1:13

syam

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4 Comments

saccharine Over a year ago

Thanks. Just found the relevant question on SO too. I figured CentOS 6 would have lambda support in gcc, but I guess not.

syam Over a year ago

@saccharine I don't know much about CentOS but you probably could install a newer GCC version from a "testing" repository rather than the regular "stable" repository (sorry for the Debian wording ;) you get my point anyway). Eg. I have g++ 4.7 and 4.8 running on Squeeze (Debian stable, GCC 4.4 by default) with only very minimal upgrades.

4pie0 Over a year ago

is the same issue with std::addressof? I am using g++4.4 because CUDA requires it, but my program doesn't see std::addressof, I see that in 4.4 files this is not present, only in my 4.7 dirs

syam Over a year ago

@cf16 it seems that your question already contains the answer. ;)

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