diff --git a/src/arch/arm/kvm/gic.cc b/src/arch/arm/kvm/gic.cc
index 887fa8d..d2423a6 100644
--- a/src/arch/arm/kvm/gic.cc
+++ b/src/arch/arm/kvm/gic.cc
@@ -305,7 +305,7 @@
 MuxingKvmGic::copyBankedDistRange(BaseGicRegisters* from, BaseGicRegisters* to,
                                   Addr daddr, size_t size)
 {
-    for (int ctx = 0; ctx < system._numContexts; ++ctx)
+    for (int ctx = 0; ctx < system.numContexts(); ++ctx)
         for (auto a = daddr; a < daddr + size; a += 4)
             copyDistRegister(from, to, ctx, a);
 }
@@ -314,7 +314,7 @@
 MuxingKvmGic::clearBankedDistRange(BaseGicRegisters* to,
                                    Addr daddr, size_t size)
 {
-    for (int ctx = 0; ctx < system._numContexts; ++ctx)
+    for (int ctx = 0; ctx < system.numContexts(); ++ctx)
         for (auto a = daddr; a < daddr + size; a += 4)
             to->writeDistributor(ctx, a, 0xFFFFFFFF);
 }
@@ -345,7 +345,7 @@
     // Copy CPU Interface Control Register (CTLR),
     //      Interrupt Priority Mask Register (PMR), and
     //      Binary Point Register (BPR)
-    for (int ctx = 0; ctx < system._numContexts; ++ctx) {
+    for (int ctx = 0; ctx < system.numContexts(); ++ctx) {
         copyCpuRegister(from, to, ctx, GICC_CTLR);
         copyCpuRegister(from, to, ctx, GICC_PMR);
         copyCpuRegister(from, to, ctx, GICC_BPR);
@@ -423,7 +423,7 @@
     // have been shifted by three bits due to its having been emulated by
     // a VGIC with only 5 PMR bits in its VMCR register.  Presently the
     // Linux kernel does not repair this inaccuracy, so we correct it here.
-    for (int cpu = 0; cpu < system._numContexts; ++cpu) {
+    for (int cpu = 0; cpu < system.numContexts(); ++cpu) {
        cpuPriority[cpu] <<= 3;
        assert((cpuPriority[cpu] & ~0xff) == 0);
     }
diff --git a/src/arch/arm/linux/system.cc b/src/arch/arm/linux/system.cc
index 7f06475..094e4d7 100644
--- a/src/arch/arm/linux/system.cc
+++ b/src/arch/arm/linux/system.cc
@@ -250,8 +250,7 @@
         std::string task_filename = "tasks.txt";
         taskFile = simout.create(name() + "." + task_filename);
 
-        for (int i = 0; i < _numContexts; i++) {
-            ThreadContext *tc = threadContexts[i];
+        for (const auto tc : threadContexts) {
             uint32_t pid = tc->getCpuPtr()->getPid();
             if (pid != BaseCPU::invldPid) {
                 mapPid(tc, pid);
diff --git a/src/sim/system.cc b/src/sim/system.cc
index ed01e0e..38eed1c 100644
--- a/src/sim/system.cc
+++ b/src/sim/system.cc
@@ -47,6 +47,8 @@
 
 #include "sim/system.hh"
 
+#include <algorithm>
+
 #include "arch/remote_gdb.hh"
 #include "arch/utility.hh"
 #include "base/loader/object_file.hh"
@@ -87,7 +89,6 @@
 
 System::System(Params *p)
     : MemObject(p), _systemPort("system_port", this),
-      _numContexts(0),
       multiThread(p->multi_thread),
       pagePtr(0),
       init_param(p->init_param),
@@ -256,7 +257,6 @@
              "Cannot have two CPUs with the same id (%d)\n", id);
 
     threadContexts[id] = tc;
-    _numContexts++;
 
 #if THE_ISA != NULL_ISA
     int port = getRemoteGDBPort();
@@ -287,12 +287,13 @@
 int
 System::numRunningContexts()
 {
-    int running = 0;
-    for (int i = 0; i < _numContexts; ++i) {
-        if (threadContexts[i]->status() != ThreadContext::Halted)
-            ++running;
-    }
-    return running;
+    return std::count_if(
+        threadContexts.cbegin(),
+        threadContexts.cend(),
+        [] (ThreadContext* tc) {
+            return tc->status() != ThreadContext::Halted;
+        }
+    );
 }
 
 void
diff --git a/src/sim/system.hh b/src/sim/system.hh
index 5b0c178..a72f2a7 100644
--- a/src/sim/system.hh
+++ b/src/sim/system.hh
@@ -196,7 +196,6 @@
 #endif
 
     std::vector<ThreadContext *> threadContexts;
-    int _numContexts;
     const bool multiThread;
 
     ThreadContext *getThreadContext(ContextID tid)
@@ -204,11 +203,7 @@
         return threadContexts[tid];
     }
 
-    int numContexts()
-    {
-        assert(_numContexts == (int)threadContexts.size());
-        return _numContexts;
-    }
+    unsigned numContexts() const { return threadContexts.size(); }
 
     /** Return number of running (non-halted) thread contexts in
      * system.  These threads could be Active or Suspended. */