View Compilation Error

0_0_17869833_25425.cpp: In constructor 'BigNum::BigNum()':
0_0_17869833_25425.cpp:17:44: error: 'memset' was not declared in this scope
  BigNum() { len = 1; memset(a, 0, sizeof(a)); }   //构造函数
                                            ^
0_0_17869833_25425.cpp: In constructor 'BigNum::BigNum(int)':
0_0_17869833_25425.cpp:42:24: error: 'memset' was not declared in this scope
  memset(a, 0, sizeof(a));
                        ^
0_0_17869833_25425.cpp: In constructor 'BigNum::BigNum(const char*)':
0_0_17869833_25425.cpp:54:24: error: 'memset' was not declared in this scope
  memset(a, 0, sizeof(a));
                        ^
0_0_17869833_25425.cpp:55:14: error: 'strlen' was not declared in this scope
  l = strlen(s);
              ^
0_0_17869833_25425.cpp: In copy constructor 'BigNum::BigNum(const BigNum&)':
0_0_17869833_25425.cpp:74:24: error: 'memset' was not declared in this scope
  memset(a, 0, sizeof(a));
                        ^
0_0_17869833_25425.cpp: In member function 'BigNum& BigNum::operator=(const BigNum&)':
0_0_17869833_25425.cpp:82:24: error: 'memset' was not declared in this scope
  memset(a, 0, sizeof(a));
                        ^
0_0_17869833_25425.cpp: In function 'std::istream& operator>>(std::istream&, BigNum&)':
0_0_17869833_25425.cpp:92:19: error: 'strlen' was not declared in this scope
  int l = strlen(ch);
                   ^