std::cout reading garbage from a vector when printf reads properly?

Question

In this code:

// read bootrom
std::ifstream bootrom_file (bootrom_path, std::ios::binary | std::ios::ate);
const int bootrom_size = bootrom_file.tellg();
bootrom_file.seekg(0, std::ios_base::beg);
// allocate bootrom_size bytes for the bootrom vector
bootrom.resize(bootrom_size);
if(bootrom_size != 0x100)
{
    std::cerr << "boot ROM is not 256 bytes!\n";
}
if(bootrom_file)
{
    bootrom_file.read(reinterpret_cast<char*>(bootrom.data()), bootrom_size);
}
// prints 0xC3 0x31
printf("%#02x  %#02x\n", rom[0], bootrom[0]);
// prints ?  1
std::cout << std::hex << rom[0] << "  " << std::hex << bootrom[0] << "\n";

std::cout prints out ? 1, but printf prints out 0xC3 0x31 which is correct. What am I doing wrong here?

Note that rom and bootrom are both std::vector of uint8_t, and rom is set using the same code as bootrom.

It is likely you have something like `typedef unsigned char uint8_t;` so `cout` prints characters. — AlexD, Mar 26 '17 at 15:43
I don't have it, uint8_t is defined in the standard (albeit as a char, yes). And I just learned that this is the reason, thanks. It prints out the ascii character. — Ryan Terry, Mar 26 '17 at 15:47
The standard does not specify `uint8_t` precisely. It says: `typedef unsigned integer type uint8_t; // optional` — AlexD, Mar 26 '17 at 15:49
I copy-pasted the `typedef` above from the standard. It is not precisely defined (and even optional). — AlexD, Mar 26 '17 at 16:00
[cout not printing unsigned char](https://stackoverflow.com/q/15585267/995714), [std::cout deal with uint8_t as a character](https://stackoverflow.com/q/39145753/995714), [uint8_t can't be printed with cout](https://stackoverflow.com/q/19562103/995714), [Does the unary + operator have any practical use?](https://stackoverflow.com/a/14365849/995714) — phuclv, Sep 03 '18 at 10:09
Possible duplicate of [uint8\_t can't be printed with cout](https://stackoverflow.com/questions/19562103/uint8-t-cant-be-printed-with-cout) — phuclv, Sep 03 '18 at 10:09
I think you need to `bootrom_file.seekg(0, std::ios_base::end);` when calculating `bootrom_size`. The call to `bootrom_file.seekg(0, std::ios_base::beg);` is still needed after `tellg()`. — jww, Sep 03 '18 at 12:03

Ryan Terry · Answer 1 · 2017-03-26T19:58:48.033

2

Solution: I figured it out. Turns out that std::cout cannot print uint8_t by value since it is a typedef char, so it prints the ascii character instead.

edited Mar 26 '17 at 19:58

answered Mar 26 '17 at 15:48

Ryan Terry

181
8

Just cast it to something which is printed as a number. – Christian Hackl Mar 26 '17 at 16:42
Obviously I did that already, which is why I just announced what the original problem was; i.e. I solved it. – Ryan Terry Mar 26 '17 at 19:57

std::cout reading garbage from a vector when printf reads properly?

1 Answers1