注意：这与在编译时确定整型中的位数非常相似，但是这是一个简化得多的版本，都包含在一个.cpp

编辑：添加了一个解决方案 - 虽然给出了正确的解释(并接受了(，但我找到了一种普遍解决问题的方法。

问题

问题在于像这样的函数

 msg(int32_t);
 msg(int64_t);

像这样的电话

long long myLong = 6;
msg(myLong);    // Won't compile on gcc (4.6.3), call is ambiguous

这将在 MSVC 上编译。谁能解释为什么这在 gcc 上失败(我假设这可能与 gcc 通常严格遵守标准的事实有关(以及如何正确实现相同效果的示例？

#include <iostream>
#include <stdint.h>
#include <boost/integer.hpp>
using namespace std;
void msg(int v) { cout << "int: " << sizeof(int) << ' ' << v << 'n'; }
void msg(long v) { cout << "long: " << sizeof(long) << ' ' << v << 'n'; }
void msg(long long v) { cout << "long long: " << sizeof(long long) << ' ' << v << 'n'; }
void msg2(int32_t v) { cout << "int32_t: " << sizeof(int32_t) << ' ' << v << 'n'; }
void msg2(int64_t v) { cout << "int64_t: " << sizeof(int64_t) << ' ' << v << 'n'; }
void msg2(uint32_t v) { cout << "uint32_t: " << sizeof(uint32_t) << ' ' << v << 'n'; }
void msg2(uint64_t v) { cout << "uint64_t: " << sizeof(uint64_t) << ' ' << v << 'n'; }

int main()
{
    int myInt = -5;
    long myLong = -6L;
    long long myLongLong = -7LL;
    unsigned int myUInt = 5;
    unsigned int myULong = 6L;
    unsigned long long myULongLong = 7LL;
    msg(myInt);
    msg(myLong);
    msg(myLongLong);
    msg2(myInt);
    msg2(myLong);     // fails on gcc 4.6.3 (32 bit)
    msg2(myLongLong);
    msg2(myUInt);
    msg2(myULong);   // fails on gcc 4.6.3 (32 bit)
    msg2(myULongLong);
   return 0;
}
// Output from MSVC  (and gcc if you omit lines that would be commented out)
int: 4 5
long: 4 6
long long: 8 7
int32_t: 4 -5
int32_t: 4 -6   // omitted on gcc
int64_t: 8 -7
uint32_t: 4 5
uint32_t: 4 6   // omitted on gcc
uint64_t: 8 7

溶液

解决方案是提供一个功能，成功地将int、long和long long映射到适当的int32_t或int64_t。这可以在运行时通过 if (sizeof(int)==sizeof(int32_t)) 类型语句轻松完成，但最好使用编译时解决方案。编译时解决方案可通过使用 boost::enable_if .

以下内容适用于 MSVC10 和 gcc 4.6.3。可以通过禁用非整型来进一步增强解决方案，但这超出了此问题的范围。

#include <iostream>
#include <stdint.h>
#include <boost/integer.hpp>
#include <boost/utility/enable_if.hpp>
#include <boost/type_traits/is_signed.hpp>
#include <boost/type_traits/is_unsigned.hpp>
using namespace std;
template <class InputT>
typename boost::enable_if_c<sizeof(InputT)==sizeof(int32_t) && boost::is_signed<InputT>::value,
 int32_t>::type ConvertIntegral(InputT z) { return static_cast<int32_t>(z); }
template <class InputT>
typename boost::enable_if_c<sizeof(InputT)==sizeof(int64_t) && boost::is_signed<InputT>::value, 
int64_t>::type ConvertIntegral(InputT z) { return static_cast<int64_t>(z); }
template <class InputT>
typename boost::enable_if_c<sizeof(InputT)==sizeof(uint32_t) && boost::is_unsigned<InputT>::value, 
uint32_t>::type ConvertIntegral(InputT z) { return static_cast<uint32_t>(z); }
template <class InputT>
typename boost::enable_if_c<sizeof(InputT)==sizeof(uint64_t) && boost::is_unsigned<InputT>::value, 
uint64_t>::type ConvertIntegral(InputT z) { return static_cast<uint64_t>(z); }
void msg(int v) { cout << "int: " << sizeof(int) << ' ' << v << 'n'; }
void msg(long v) { cout << "long: " << sizeof(long) << ' ' << v << 'n'; }
void msg(long long v) { cout << "long long: " << sizeof(long long) << ' ' << v << 'n'; }

void msg2(int32_t v) { cout << "int32_t: " << sizeof(int32_t) << ' ' << v << 'n'; }
void msg2(int64_t v) { cout << "int64_t: " << sizeof(int64_t) << ' ' << v << 'n'; }
void msg2(uint32_t v) { cout << "uint32_t: " << sizeof(uint32_t) << ' ' << v << 'n'; }
void msg2(uint64_t v) { cout << "uint64_t: " << sizeof(uint64_t) << ' ' << v << 'n'; }
int main()
{
    int myInt = -5;
    long myLong = -6L;
    long long myLongLong = -7LL;
    unsigned int myUInt = 5;
    unsigned int myULong = 6L;
    unsigned long long myULongLong = 7LL;
    msg(myInt);
    msg(myLong);
    msg(myLongLong);
    msg2(ConvertIntegral(myInt));
    msg2(ConvertIntegral(myLong));
    msg2(ConvertIntegral(myLongLong));
    msg2(ConvertIntegral(myUInt));
    msg2(ConvertIntegral(myULong));
    msg2(ConvertIntegral(myULongLong));
   return 0;
}