c++ system call raises ENOMEM

This question is a M(not)WE of this question. I wrote a code that reproduces the error :

#include <cstdlib>
#include <iostream>
#include <vector>

int *watch_errno = __errno_location();

int main(){
    std::vector<double> a(7e8,1);
    std::cout<<system(NULL)<<std::endl;
}

It has to be compiled with g++ -ggdb -std=c++11 (g++ 4.9 on a Debian). Note that the int *watch_errno is only usefull to allow gdb to watch errno. When it is run trough gdb, I get this :

GNU gdb (Debian 7.7.1+dfsg-5) 7.7.1
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Reading symbols from bug...done.
(gdb) watch *watch_errno 
Hardware watchpoint 1: *watch_errno
(gdb) r
Starting program: /tmp/bug 
Hardware watchpoint 1: *watch_errno

Old value = <unreadable>
New value = 0
__static_initialization_and_destruction_0 (__initialize_p=1, __priority=65535) at bug.cpp:10
10      }
(gdb) c
Continuing.
Hardware watchpoint 1: *watch_errno

Old value = 0
New value = 12
0x00007ffff7252421 in do_system (line=line@entry=0x7ffff7372168 "exit 0") at ../sysdeps/posix/system.c:116
116     ../sysdeps/posix/system.c: No such file or directory.
(gdb) bt
#0  0x00007ffff7252421 in do_system (line=line@entry=0x7ffff7372168 "exit 0") at ../sysdeps/posix/system.c:116
#1  0x00007ffff7252510 in __libc_system (line=<optimized out>) at ../sysdeps/posix/system.c:182
#2  0x0000000000400ad8 in main () at bug.cpp:9
(gdb) l
111     in ../sysdeps/posix/system.c
(gdb) c
Continuing.
0
[Inferior 1 (process 5210) exited normally]

For some reason errno is set to ENOMEM at line 9 which corresponds to the system call. Note that if the vector has a smaller size (I guess that it depends on which computer you'll run the code), the code works fine and system(NULL) returns 1 as it should when a shell is available.

Why is the flag ENOMEM raised ? Why isn't the code using the SWAP memory ? Is this a bug ? Is there a workaround ? Would popen or exec* do the same ? (I know, I should only ask one question per post, but all these question could be summarized by : "what is going on ?")