C/C++ :: How To Clear Nested Stl Map

here is a nested stl map

typedef struct struct_RISCP {    
        float nSCPTotal;
        int nSCPCount;
        CString strIMEI;  
        std::map<UINT8, int> mapSection;    
    }RISCPSt;
map<stRncCellIdntyDmnType, RISCPSt>m_mapRISCP;

it occupied too much memory,i wanted to clear them(both outer map and inner map) ,some one told me you just need to call m_mapRISCP.clear(),then the mapSection (inner map) will be cleared automaticly,in other words, m_mapRISCP.clear() will clear both outer map and inner map.