View Compilation Error

0_0_13797206_12298.cpp: In constructor 'BigNum::BigNum()':
0_0_13797206_12298.cpp:19:40: error: 'memset' was not declared in this scope
  BigNum(){ len = 1;memset(a,0,sizeof(a)); }   //构造函数
                                        ^
0_0_13797206_12298.cpp: In constructor 'BigNum::BigNum(int)':
0_0_13797206_12298.cpp:44:22: error: 'memset' was not declared in this scope
  memset(a,0,sizeof(a));
                      ^
0_0_13797206_12298.cpp: In constructor 'BigNum::BigNum(const char*)':
0_0_13797206_12298.cpp:56:22: error: 'memset' was not declared in this scope
  memset(a,0,sizeof(a));
                      ^
0_0_13797206_12298.cpp:57:12: error: 'strlen' was not declared in this scope
  l=strlen(s);   
            ^
0_0_13797206_12298.cpp: In copy constructor 'BigNum::BigNum(const BigNum&)':
0_0_13797206_12298.cpp:76:22: error: 'memset' was not declared in this scope
  memset(a,0,sizeof(a)); 
                      ^
0_0_13797206_12298.cpp: In member function 'BigNum& BigNum::operator=(const BigNum&)':
0_0_13797206_12298.cpp:84:22: error: 'memset' was not declared in this scope
  memset(a,0,sizeof(a)); 
                      ^
0_0_13797206_12298.cpp: In function 'std::istream& operator>>(std::istream&, BigNum&)':
0_0_13797206_12298.cpp:94:17: error: 'strlen' was not declared in this scope
  int l=strlen(ch);
                 ^