Why does std::hex lead to memory corruption vector.size()?

Question

I have following code:

vector<UINT_PTR> test = GetMemoryAddresses();

cout << "Size: " << test.size() << endl;

for (UINT_PTR a : test) {
    cout << "Memory Address: " << hex  << a << endl;
}

cout << "Size: " << test.size() << endl;

Which prints following results:

Size: 18
Memory Address: fc06a0
Memory Address: 13a70f0
Memory Address: 36c78c1
Memory Address: 3da0ea0
Memory Address: 3e21b80
Memory Address: c0a6881
Memory Address: c747690
Memory Address: c748b98
Memory Address: c74a1b8
Memory Address: c74ce10
Memory Address: c750c78
Memory Address: 1340a10f
Memory Address: 14376ea0
Memory Address: 14472649
Memory Address: 69867268
Memory Address: 6bdf9243
Memory Address: 7399a142
Memory Address: 76c54875
Size: 12

Removing the

<< hex

leads to the correct vector.size() of 18 again.

What is causing this memory coruption?

Have I done anything wrong?

[Related question](https://stackoverflow.com/questions/2273330/restore-the-state-of-stdcout-after-manipulating-it) on saving and restoring a stream's state. — François Andrieux, Feb 20 '19 at 14:37
This is because `` and `` are [completely broken](https://stackoverflow.com/questions/52442415/in-c-am-i-paying-for-what-i-am-not-eating/52462699#52462699). You should just include `` and use `std::fprintf()`. — KevinZ, Feb 21 '19 at 04:02

NathanOliver · Accepted Answer · 2019-02-20T14:49:04.170

10

std::hex is "sticky". It sets the state of the stream to hexadecimal display and that is where it will stay until you change it. That means when you do

cout << "Size: " << test.size() << endl;

you display the size in hex and 18 decimal is 12 hex.

You need

cout << "Size: " << dec << test.size() << endl;

to switch back to decimal mode and display the size "correctly".

Another thing that would help you diagnosing this is to use std::showbase. It is another "sticky" manipulator and will prepend hexadecimal output with 0x. Using that in

cout << "Memory Address: " << hex << showbase  << a << endl;

would have made your output

Size: 18
Memory Address: 0xfc06a0
Memory Address: 0x13a70f0
Memory Address: 0x36c78c1
Memory Address: 0x3da0ea0
Memory Address: 0x3e21b80
Memory Address: 0xc0a6881
Memory Address: 0xc747690
Memory Address: 0xc748b98
Memory Address: 0xc74a1b8
Memory Address: 0xc74ce10
Memory Address: 0xc750c78
Memory Address: 0x1340a10f
Memory Address: 0x14376ea0
Memory Address: 0x14472649
Memory Address: 0x69867268
Memory Address: 0x6bdf9243
Memory Address: 0x7399a142
Memory Address: 0x76c54875
Size: 0x12

which makes it a lot more clear that the last call to cout is still using hex

edited Feb 20 '19 at 14:49

answered Feb 20 '19 at 14:36

NathanOliver

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Jesus christ why is C++ like that. Thanks <3 – Ojav Feb 20 '19 at 14:37
@Ojav I cannot wait for fmt to be standardized. – Guillaume Racicot Feb 20 '19 at 14:38
2

@Ojav Don't worry about it. Some manipulators are not "sticky" and some are. It takes time to learn which is which. – NathanOliver Feb 20 '19 at 14:38
@Ojav it really isnt obvious by reading some otherwise wonderful [reference](https://en.cppreference.com/w/cpp/io/manip/hex), though if you read the example carefully they mention: "// the output base is sticky until changed" – 463035818_is_not_an_ai Feb 20 '19 at 14:38
TIL. Confusing business with most common stream manipulators not sticking. – StoryTeller - Unslander Monica Feb 20 '19 at 14:39
@user463035818 Men i even was at that site, I never felt more stupid ever – Ojav Feb 20 '19 at 14:42
@Ojav Don't worry about it. Stream manipulators are tricky like that, and everyone makes mistakes like this once in a while. – François Andrieux Feb 20 '19 at 14:45
@Ojav I've updated the answer with a useful debug tip for this. – NathanOliver Feb 20 '19 at 14:49
@Ojav not your fault, if I didnt miss it they dont mention it at all in the text, maybe it should get an update – 463035818_is_not_an_ai Feb 20 '19 at 14:59

Alan Birtles · Answer 2 · 2019-02-20T14:51:05.313

6

std::cout << std::hex;

makes all numbers printed by std::cout print as hex. To get back to decimal again run:

std::cout << std::dec;

Ideally before using any stream format manipulators (like std::hex) you should save the formatting flags and restore them when you are finished. For example:

auto flags = std::cout.flags();
std::cout << std::hex << 10 << "\n";
std::cout.flags( flags );

All of the flags need to be reset like this: https://en.cppreference.com/w/cpp/io/ios_base/flags

The fill character is also sticky: https://en.cppreference.com/w/cpp/io/basic_ios/fill and so is the precision: https://en.cppreference.com/w/cpp/io/ios_base/precision

The only one that isn't is the width flag: https://en.cppreference.com/w/cpp/io/ios_base/width

Boost has a utility class to reset all the stream state back to the initial values: https://www.boost.org/doc/libs/1_69_0/libs/io/doc/ios_state.html

edited Feb 20 '19 at 14:51

answered Feb 20 '19 at 14:34

Alan Birtles

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@Ojav it didnt change, 0x12 is 18 – 463035818_is_not_an_ai Feb 20 '19 at 14:36
Thanks to you too kind sir – Ojav Feb 20 '19 at 14:39
@StoryTeller nope, `setw` resets on the next output operation but the other flags don't – Alan Birtles Feb 20 '19 at 14:40

Why does std::hex lead to memory corruption vector.size()?

2 Answers2