memset to struct 引起的 core

#include<string.h>
using namespace std;

struct A
{
    int a;
    int b;
};

struct B
{
    int a;
    char aa[10];
    char ab[30];
    int b;
    char ni[40];
    string cs; 
};

int main()
{
    /*A mya;

    mya.a=3;
    mya.b=4;
    void * pa =(void*)&mya;

    B* pb=(B*)pa;*/

    B ba; 
    memset(&ba,0,sizeof(B));
    /*cout<<sizeof(mya)<<endl;
    cout<<sizeof(B)<<endl;
    cout<<pb->a<<endl;
    cout<<pb->b<<endl;
    cout<<pb->aa<<endl;
    cout<< pb->ni<<endl;*/
    //cout<< pb->cs;
}

(gdb) bt
#0  0x00007ff0978960d7 in __exchange_and_add (this=0xffffffffffffffe8, __a=...)
    at /data/home/root/source/build_gcc4.7.4/x86_64-unknown-linux-gnu/libstdc++-v3/include/ext/atomicity.h:48
#1  __exchange_and_add_dispatch (this=0xffffffffffffffe8, __a=...)
    at /data/home/root/source/build_gcc4.7.4/x86_64-unknown-linux-gnu/libstdc++-v3/include/ext/atomicity.h:81
#2  std::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Rep::_M_dispose (this=0xffffffffffffffe8, __a=...)
    at /data/home/root/source/build_gcc4.7.4/x86_64-unknown-linux-gnu/libstdc++-v3/include/bits/basic_string.h:242
#3  0x00007ff097896130 in std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string (this=Unhandled dwarf expression opcode 0xf3
)
    at /data/home/root/source/build_gcc4.7.4/x86_64-unknown-linux-gnu/libstdc++-v3/include/bits/basic_string.h:535
#4  0x0000000000400c00 in B::~B (this=0x7fff35938f40, __in_chrg=<value optimized out>) at test_struct.cpp:13
#5  0x0000000000400b63 in main () at test_struct.cpp:39