View Compilation Error

0_0_1645030_14026.cpp: In function `std::string add_str(std::string, std::string)':
0_0_1645030_14026.cpp:51: error: `itoa' undeclared (first use this function)
0_0_1645030_14026.cpp:51: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_1645030_14026.cpp: In function `std::string multiple(std::string, int)':
0_0_1645030_14026.cpp:78: error: `itoa' undeclared (first use this function)