问题描述
静态常量字段初始化顺序有没有陷阱?
Is there any trap in static constant field initialization order?
template <typename T>
struct constant_test {
static const T PI;
static const T FULL_CIRCLE;
static const T HALF_CIRCLE;
static const T DEG_TO_RAD;
};
template <typename T> const T constant_test<T>::PI = 3.141592653589f;
template <typename T> const T constant_test<T>::FULL_CIRCLE = 360.0f;
template <typename T> const T constant_test<T>::HALF_CIRCLE = constant_test<T>::FULL_CIRCLE / 2;
template <typename T> const T constant_test<T>::DEG_TO_RAD = constant_test<T>::PI / constant_test<T>::HALF_CIRCLE;
int APIENTRY _tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow) {
// uncomment to make it work
// float test_ref = constant_test<float>::HALF_CIRCLE;
char buf[128];
sprintf_s(buf, 128, "Value: %f", constant_test<float>::DEG_TO_RAD);
OutputDebugStringA(buf); // prints "Value: 1.#INF00"
return 0;
}
一个表达式 constant_test::DEG_TO_RAD 神奇地返回 -Infinity
An expression constant_test<float>::DEG_TO_RAD magically returns -Infinity
如果我删除模板参数并使它们只浮动，那么常量将被正确评估 (0.017453)
If i remove template parameter and make them only float then constant is evaluated correctly (0.017453)
如果我添加对 HALF_CIRCLE 常量的引用，那么它也会被正确评估
If i add reference to HALF_CIRCLE constant then it's also evaluated correctly
我使用的是 MSVC 2013 SP 1.
I'm using MSVC 2013 SP 1.
为什么?我错过了什么?
Why?What i'm missing?
推荐答案
此代码打印出正确的值 (g++4.8.2, OS X):
This code prints out the correct value (g++4.8.2, OS X):
#include <iostream>
using namespace std;
template <typename T>
struct constant_test {
static const T PI;
static const T FULL_CIRCLE;
static const T HALF_CIRCLE;
static const T DEG_TO_RAD;
};
template <typename T> const T constant_test<T>::PI = 3.141592653589f;
template <typename T> const T constant_test<T>::FULL_CIRCLE = 360.0f;
template <typename T> const T constant_test<T>::HALF_CIRCLE = constant_test<T>::FULL_CIRCLE / 2;
template <typename T> const T constant_test<T>::DEG_TO_RAD = constant_test<T>::PI / constant_test<T>::HALF_CIRCLE;
int main()
{
// uncomment to make it work
//float test_ref = constant_test<float>::HALF_CIRCLE;
cout << "Value: " << constant_test<float>::DEG_TO_RAD << endl; // correct result
char buf[128];
sprintf(buf, "Value: %f", constant_test<float>::DEG_TO_RAD );
cout << buf << endl; // again correct
return 0;
}
输出:
Value: 0.0174533
Value: 0.017453
静态常量按顺序初始化，见https://stackoverflow.com/a/10011133/3093378，所以你应该在显示它时初始化 constant_test<T>::DEG_TO_RAD (从上面的代码看起来是这样).
The static constants are initialized in order, see https://stackoverflow.com/a/10011133/3093378, so you should have constant_test<T>::DEG_TO_RAD initialized when you display it (and it looks it is, from the code above).
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