Valgrind showing unexpected output in C++ code

Question

In the beginning of a C++ code, I initialize a vector of 1000000(a million) bool type data. However, in valgrind, the maximum heap + stack usage is shown 200Kb. Given that a Bool is 1 Byte, shouldn't it be 1 Mb ?

Is there an optimization that I don't realise ? Or am I missing something ?

Thanks is advance.

I use a Ubuntu64 16.04 system. Compiling the code without -O parameter.

Edit: The code can be simplified to this,

vector<bool> * isPrime;
int main(){
    isPrime = new vector<bool>(1000000, true);
}

Edit2: It seems there was an optimization that I don't realise (which is stated in the comments). Thanks.

Related, possible duplicate: https://stackoverflow.com/q/17794569/1896169 — Justin, May 02 '18 at 22:12
and .. where and how is the bool type is created, allocated? you need to provide your code. It could be static, it could be packed, .. — Serge, May 02 '18 at 22:13
@Justin I couldn't find such a knowledge, using what I asked here. But it seems the answer, thanks. — Max Paython, May 02 '18 at 22:14
`vector` is such a weird lil' guy it gets its own page on cppreference: http://en.cppreference.com/w/cpp/container/vector_bool — user4581301, May 02 '18 at 22:21
So, why the C language tag? Your post title says C++, your post discusses C++. So where do you discuss the C language? — Thomas Matthews, May 02 '18 at 22:38
BTW, the C language does not have `std::vector`. I recommend you figure out which language you are programming in. — Thomas Matthews, May 02 '18 at 22:39
@ThomasMatthews That was to reach more people that might be checking the C tag and not C++, and still know the answer. I'll remove it if it is unsuitable or irritating — Max Paython, May 02 '18 at 22:45
@MaxPaython Don't worry, *plenty* of people check the C++ tag... — Barry, May 03 '18 at 01:39

score 0 · Answer 1 · answered May 03 '18 at 07:22

Compiling the code without -O parameter is equal to -O0, and the -O0 does not mean no optimization. You can see the define of gcc optimization in https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

In the case, the size same as -Os, and if you use -O1 or higher, the compiler does not use bit vector, in the other words, each bool varaible occupy 1 byte; otherwise the compiler choose bit vector (http://www.cplusplus.com/reference/vector/vector-bool/)

Valgrind showing unexpected output in C++ code

1 Answers1