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Asked: 2026-05-11T04:16:19+00:00

#include <iostream> #include <vector> using namespace std; int main() { vector< vector<int> > dp(50000,

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#include <iostream> #include <vector> using namespace std;  int main() {     vector< vector<int> > dp(50000, vector<int>(4, -1));     cout << dp.size(); }

This tiny program takes a split second to execute when simply run from the command line. But when run in a debugger, it takes over 8 seconds. Pausing the debugger reveals that it is in the middle of destroying all those vectors. WTF?

Note – Visual Studio 2008 SP1, Core 2 Duo 6700 CPU with 2GB of RAM.

Added: To clarify, no, I’m not confusing Debug and Release builds. These results are on one and the same .exe, without even any recompiling inbetween. In fact, switching between Debug and Release builds changes nothing.

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  1. Running in the debugger changes the memory allocation library used to one that does a lot more checking. A program that does nothing but memory allocation and de-allocation is going to suffer much more than a ‘normal’ program.

    Edit Having just tried running your program under VS I get a call stack that looks like

    ntdll.dll!_RtlpValidateHeapEntry@12()  + 0x117 bytes     ntdll.dll!_RtlDebugFreeHeap@12()  + 0x97 bytes   ntdll.dll!_RtlFreeHeapSlowly@12()  + 0x228bf bytes   ntdll.dll!_RtlFreeHeap@12()  + 0x17646 bytes     msvcr90d.dll!_free_base(void * pBlock=0x0061f6e8)  Line 109 + 0x13 bytes msvcr90d.dll!_free_dbg_nolock(void * pUserData=0x0061f708, int nBlockUse=1) msvcr90d.dll!_free_dbg(void * pUserData=0x0061f708, int nBlockUse=1)  msvcr90d.dll!operator delete(void * pUserData=0x0061f708) desc.exe!std::allocator<int>::deallocate(int * _Ptr=0x0061f708, unsigned int __formal=4) desc.exe!std::vector<int,std::allocator<int> >::_Tidy()  Line 1134  C++

    Which shows the debug functions in ntdll.dll and the C runtime being used.

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