Crash when initializing a std::string array in Visual Studio C++

Question

The problem: I am trying to allocate memory for an array of std::string , then using new(...)[]() to initialize it. Then I try to assign values to the array elements and this causes the application to crash (Access violation). Question is: Am I missing some compiler flag or something obvious?

Compiling this with

cl.exe /DEBUG /EHsc /MTd /Zi test.cc

produces an executable that crashes (tested in VS 2017 and VS 2012). On a side note it works as expected on Linux with GCC.

#include <iostream>
#include <string>

struct S {
  int a,b;
  S() : a(99), b(299) {}
  S & operator=(const char * rhs) { a = 100; b = 300; return *this; }  
};
std::ostream & operator<<(std::ostream & os, const S & s) { os << "(" << s.a << "," << s.b << ")"; return os; }  


typedef std::string T;
//typedef S T;
int main(int argc, char ** argv) {
    size_t N = 100;
    std::allocator<T> mem;
    T * data = mem.allocate(N);
    new(data)T[N]();

    for (ptrdiff_t i = 0; i < N; ++i)
        data[i] = "HELLO WORLD";
    for (ptrdiff_t i = 0; i < N; ++i)
        std::cout << data[i] << std::endl;
}

I tried using the other typedef , and in that case the initialization works exactly as expected.

EDIT: I see the same crash if I use C calloc for allocation instead of std::allocator .

SOLVED. It turns out that the problem is this: The C++ standard says that the new T[N] expression allocates at least sizeof(T)*N bytes. Visual C allocates an extra overhead. The placement new new(ptr)T[N] assumes that the extra overhead is available in ptr . The solution from the answer below is to loop and construct each item separately.

Answer 1

The issue is with the unspecified overhead as described here very well: Array placement-new requires unspecified overhead in the buffer?

A quick fix would be

#include <iostream>
#include <string>

struct S {
  int a,b;
  S() : a(99), b(299) {}
  S & operator=(const char * rhs) { a = 100; b = 300; return *this; }  
};
std::ostream & operator<<(std::ostream & os, const S & s) { os << "(" << s.a << "," << s.b << ")"; return os; }  


typedef std::string T;
//typedef S T;
int main(int argc, char ** argv) {
    size_t N = 100;
    std::allocator<T> mem;
    T * datastor = mem.allocate(N+1);
    T * data = new(datastor)T[N]();

    for (ptrdiff_t i = 0; i < N; ++i)
        data[i] = "HELLO WORLD";
    for (ptrdiff_t i = 0; i < N; ++i)
        std::cout << data[i] << std::endl;
}

But this is not guaranteed to work always since the "unspecified overhead" might theoretically be larger than sizeof(std::string) .

The version without unspecified behavior would new up the elements individually like so:

#include <iostream>
#include <string>

struct S {
  int a,b;
  S() : a(99), b(299) {}
  S & operator=(const char * rhs) { a = 100; b = 300; return *this; }  
};
std::ostream & operator<<(std::ostream & os, const S & s) { os << "(" << s.a << "," << s.b << ")"; return os; }  


typedef std::string T;
//typedef S T;
int main(int argc, char ** argv) {
    size_t N = 100;
    std::allocator<T> mem;
    T * data = mem.allocate(N);
    for (ptrdiff_t i = 0; i < N; ++i)
        new(&data[i]) T();
    for (ptrdiff_t i = 0; i < N; ++i)
        data[i] = "HELLO WORLD";
    for (ptrdiff_t i = 0; i < N; ++i)
        std::cout << data[i] << std::endl;
    for (ptrdiff_t i = 0; i < N; ++i)
        data[i].~T();
    mem.deallocate(data, N);
}

Answer 2

Aren't you supposed to use construct?

#include <iostream>
#include <string>

struct S {
  int a,b;
  S() : a(99), b(299) {}
  S & operator=(const char * rhs) { a = 100; b = 300; return *this; }
};
std::ostream & operator<<(std::ostream & os, const S & s) { os << "(" << s.a << "," << s.b << ")"; return os; }


typedef std::string T;
//typedef S T;
int main(int argc, char ** argv) {
    size_t N = 100;
    std::allocator<T> mem;
    T * data = mem.allocate(N);
    //new(data)T[N]();

    for (ptrdiff_t i = 0; i < N; ++i)
        mem.construct(data + i, "HELLO WORLD");
        //data[i] = "HELLO WORLD";
    for (ptrdiff_t i = 0; i < N; ++i)
        std::cout << data[i] << std::endl;
}

Answer 3

The claim that it "works" with GCC is something of an exaggeration:

g++ -std=c++17 -fPIC -g -Wall -Wextra -Wwrite-strings -Wno-parentheses -Wpedantic -Warray-bounds -Weffc++      16486983.cpp    -o 16486983
16486983.cpp: In member function ‘S& S::operator=(const char*)’:
16486983.cpp:7:30: warning: unused parameter ‘rhs’ [-Wunused-parameter]
   S & operator=(const char * rhs) { a = 100; b = 300; return *this; }
                              ^~~
16486983.cpp: In function ‘int main(int, char**)’:
16486983.cpp:20:10: error: ‘ptrdiff_t’ was not declared in this scope
     for (ptrdiff_t i = 0; i < N; ++i)
          ^~~~~~~~~
16486983.cpp:20:10: note: suggested alternatives:
In file included from /usr/include/c++/6/iostream:38:0,
                 from 16486983.cpp:1:
/usr/include/x86_64-linux-gnu/c++/6/bits/c++config.h:202:28: note:   ‘std::ptrdiff_t’
   typedef __PTRDIFF_TYPE__ ptrdiff_t;
                            ^~~~~~~~~
/usr/include/x86_64-linux-gnu/c++/6/bits/c++config.h:202:28: note:   ‘std::ptrdiff_t’
16486983.cpp:20:27: error: ‘i’ was not declared in this scope
     for (ptrdiff_t i = 0; i < N; ++i)
                           ^
16486983.cpp:22:10: error: ‘ptrdiff_t’ was not declared in this scope
     for (ptrdiff_t i = 0; i < N; ++i)
          ^~~~~~~~~
16486983.cpp:22:10: note: suggested alternatives:
In file included from /usr/include/c++/6/iostream:38:0,
                 from 16486983.cpp:1:
/usr/include/x86_64-linux-gnu/c++/6/bits/c++config.h:202:28: note:   ‘std::ptrdiff_t’
   typedef __PTRDIFF_TYPE__ ptrdiff_t;
                            ^~~~~~~~~
/usr/include/x86_64-linux-gnu/c++/6/bits/c++config.h:202:28: note:   ‘std::ptrdiff_t’
16486983.cpp:22:27: error: ‘i’ was not declared in this scope
     for (ptrdiff_t i = 0; i < N; ++i)
                           ^
16486983.cpp:14:14: warning: unused parameter ‘argc’ [-Wunused-parameter]
 int main(int argc, char ** argv) {
              ^~~~
16486983.cpp:14:28: warning: unused parameter ‘argv’ [-Wunused-parameter]
 int main(int argc, char ** argv) {
                            ^~~~

These need to be fixed; thankfully that's not hard:

#include <cstddef>
#include <iostream>
#include <string>

struct S {
  int a,b;
  S() : a(99), b(299) {}
  S & operator=(const char*) { a = 100; b = 300; return *this; }
};
std::ostream & operator<<(std::ostream & os, const S & s) { os << "(" << s.a << "," << s.b << ")"; return os; }


//typedef std::string T;
typedef S T;
int main() {
    std::size_t N = 100;
    std::allocator<T> mem;
    T * data = mem.allocate(N);
    new(data)T[N]();

    for (std::size_t i = 0; i < N; ++i)
        data[i] = "HELLO WORLD";
    for (std::size_t i = 0; i < N; ++i)
        std::cout << data[i] << std::endl;
}

Now it runs, almost¹ Valgrind-clean. But the naked placement-new looks strange - if you're using an allocator, you should use its construct() :

for (std::size_t i = 0; i < N; ++i)
    mem.construct(data+i);

---

¹Don't forget to release the memory:

for (std::size_t i = 0; i < N; ++i)
    mem.destroy(data+i);
mem.deallocate(data, N);