什么时候应该在C ++ 11中使用constexpr功能？

#1 楼

constexpr int MeaningOfLife ( int a, int b ) { return a * b; }

const int meaningOfLife = MeaningOfLife( 6, 7 );

template< typename Type > constexpr Type max( Type a, Type b ) { return a < b ? b : a; }

#2 楼

int f (int n) {
  return n > 0 ? n + f (n-1) : n;
}

T arr[f(10)]; // freakin' magic

#3 楼

int x[10];
int y[sizeof x / sizeof x[0]];

int y[size_of(x)];

#4 楼

// This is bad for obvious reasons.
#define ONE 1;

// This works most of the time but isn't fully typed.
enum { TWO = 2 };

// This doesn't compile
enum { pi = 3.1415f };

// This is a file local lvalue masquerading as a global
// rvalue.  It works most of the time.  But May subtly break
// with static initialization order issues, eg pi = 0 for some files.
static const float pi = 3.1415f;

// This is a true constant rvalue
constexpr float pi = 3.1415f;

// Haven't you always wanted to do this?
// constexpr std::string awesome = "oh yeah!!!";
// UPDATE: sadly std::string lacks a constexpr ctor

struct A
{
   static const int four = 4;
   static const int five = 5;
   constexpr int six = 6;
};

int main()
{
   &A::four; // linker error
   &A::six; // compiler error

   // EXTREMELY subtle linker error
   int i = rand()? A::four: A::five;
   // It not safe use static const class variables with the ternary operator!
}

//Adding this to any cpp file would fix the linker error.
//int A::four;
//int A::six;

#5 楼

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#7 楼

const std::complex<double> meaning_of_imagination(0, 42); 

#8 楼

template<unsigned T>
struct Fact {
    enum Enum {
        VALUE = Fact<T-1>*T;
    };
};

template<>
struct Fact<1u> {
    enum Enum {
        VALUE = 1;
    };
};

// Fact<10>::VALUE is known be a compile-time constant

int fact(unsigned n) {
    if (n==1) return 1;
    return fact(n-1)*n;
}

int main() {
    return fact(10);
}

int main() {
    int tab[fact(10)];
    int tab2[std::max(20,30)];
}

#9 楼

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#11 楼

static const int x = 5;
int arr[x];

int arr[5];

#12 楼

class MyInterface {
public:
    int GetNumber() const = 0;
};

class MyInterface {
public:
    constexpr int GetNumber() const = 0;
};

#13 楼

#14 楼

// constants:
const int MeaningOfLife = 42;

// constexpr-function:
constexpr int MeaningOfLife () { return 42; }

int some_arr[MeaningOfLife()];