me viennent à l'esprit ..
1) soit système: récupération de l'info par top/ps
peut être pas le mieux pour la portabilité
2) soit dans le code
avec une variable globale et à chaque fois que tu alloues/dé-alloues
quelque chose tu incrémentes-décrémentes la variable de la taille de ce que tu alloues/dé-alloues ...

$ man getrusage

GETRUSAGE(2) Linux GETRUSAGE(2)

NAME
getrusage - get resource usage

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 */
};

Voir:

mtrace
valgrind
mudflap
purify (payant)

Autre possibilite: surchager new/delete pour les classes suspectees de fuites.