C++ simple question

I am using uVision 4.21 MDK_ARM to learn how to program C++ on MCB1700.

I have create a new project and add 2 files (startup_CPL17xx.s / system_LPCxx.c) to startup group. Then I add main.cpp to Sources Files group. The project options are:

Device: LPC 1768
Target: Default
Output: Default
Listing: Default
User: Default
C++: Default
Asm: Default
Linker: Default
Debug: Use ULINK Cortex Debugger
Utilities: Default

Then I change the startup_LPC17xx.s
Stack Size: 0x0000 1000
Heap Size: 0x0000 1000

My main.cpp code:

#include <vector>
#include <string>

int main (void)
{
  int i = 0;
  int sum = 0;
  std::vector<int> vInt;
  std::string str = "Hello, C++";

  vInt.push_back(1);
  vInt.push_back(2);
  vInt.push_back(3);
  vInt.push_back(4);
  vInt.push_back(5);

  sum = vInt.size();
  sum = 0;
  for(i=0; i<5; i++)
  {
    sum += vInt[i];
  }
  while(1);
  //return 0;
}

I build the project successfully and download it to MCB1700 board "OK". Then I hit "F9" to set break point at line "vInt.push_back(1);". After I select Debug->Start Debug Session, the break point is not reached even I hit "F5" many times. Why?

By the way, I find there are 2 windows: Disassembly Window and Code Window (for main.cpp). How could I step through the code on C++ source code level and how could I step through the code on Assembly level?