问题：我正在尝试为std::string数组分配内存，然后使用new(...)[]()对其进行初始化。然后我尝试为数组元素赋值，这会导致应用程序崩溃（访问冲突）。问题是：我是否遗漏了一些编译器标志或明显的东西？
使用
进行编译cl.exe /DEBUG /EHsc /MTd /Zi test.cc
生成崩溃的可执行文件（在VS 2017和VS 2012中测试）。另一方面，它在Linux上使用GCC正常工作。
#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;
}
我尝试使用其他typedef，在这种情况下，初始化完全按预期工作。
编辑：如果我使用C calloc代替std::allocator，我会看到相同的崩溃。
解决。事实证明问题是：C ++标准说new T[N]表达式至少 sizeof(T)*N字节分配。 Visual C分配额外的开销。展示位置新new(ptr)T[N]假设ptr中有额外开销。
下面答案的解决方案是分别循环和构建每个项目。
答案 0 :(得分：1)
问题在于未完成的开销，如此处所述：Array placement-new requires unspecified overhead in the buffer?
快速修复
#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;
}
但这并不能保证始终有效，因为理论上“未指定的开销”可能大于sizeof(std::string)。
没有未指定行为的版本将单独新建元素：
#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);
}
答案 1 :(得分：0)
你不应该使用构造吗？
#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;
}
答案 2 :(得分：0)
声称它与海湾合作委员会“有效”是夸张的说法：
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) {
^~~~
这些需要修复;谢天谢地，这并不难：
#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;
}
现在它运行了，差不多是Valgrind-clean。但裸露的位置看起来很奇怪 - 如果你使用的是分配器，你应该使用它的construct()：
for (std::size_t i = 0; i < N; ++i)
mem.construct(data+i);
for (std::size_t i = 0; i < N; ++i)
mem.destroy(data+i);
mem.deallocate(data, N);