printf 整数显示比 C++ cout 多

Question

I'm a C programmer picking up C++

Seems cout is a bit clunky, I don't understand why it produces a different output.

//The printf below displays value 50
//The cout below display value 2

data_to_send.data[2] = atoi(data_str.c_str());    
printf("\n DEBUG: %d\n",data_to_send.data[2]);
std::cout << "Debug: " << std::dec << data_to_send.data[2];

Answer 1

Your data_to_send.data[2] field is a char or unsigned char type (byte).

printf sees it as the integer 50 , as requested by the format specifier %d .

cout sees it as the character '2' , of ASCII code 50 , as implictly requested by the data type.

Answer 2

My guess is that data_to_send.data is char* , so data_to_send.data[2] is char with value 50 . When you call printf you specify to treat your argument as integer ( %d ) so it prints 50 . In C++ cout uses function overloading to call concrete function to concrete type of argument, in this case operator<< for char . ASCII value of '2' is 50 , thats why 2 is printed out.