How to measure memory usage from inside a C++ program?

Yes – use POSIX getrusage. From the Linux man page:

Synopsis

#include <sys/time.h>
#include <sys/resource.h>

int getrusage(int who, struct rusage *usage);

Description

getrusage() returns current resource usages, for a who of either RUSAGE_SELF or RUSAGE_CHILDREN. The former asks for resources used by the current process, the latter for resources used by those of its children that have terminated and have been waited for.

struct rusage {
    struct timeval ru_utime; /* user time used */
    struct timeval ru_stime; /* system time used */
    long   ru_maxrss;        /* maximum resident set size */
    long   ru_ixrss;         /* integral shared memory size */
    long   ru_idrss;         /* integral unshared data size */
    long   ru_isrss;         /* integral unshared stack size */
    long   ru_minflt;        /* page reclaims */
    long   ru_majflt;        /* page faults */
    long   ru_nswap;         /* swaps */
    long   ru_inblock;       /* block input operations */
    long   ru_oublock;       /* block output operations */
    long   ru_msgsnd;        /* messages sent */
    long   ru_msgrcv;        /* messages received */
    long   ru_nsignals;      /* signals received */
    long   ru_nvcsw;         /* voluntary context switches */
    long   ru_nivcsw;        /* involuntary context switches */
};