在 static_assert() 编译时显示整数 [英] Display integer at compile time in static_assert()

问题描述

这是我正在尝试做的简化版本

Here is a simplified version of what I'm trying to do

enum First
{
    a,
    b,
    c,
    nbElementFirstEnum,
};
enum Second
{
    a,
    b,
    c,
    nbElementSecondEnum,
};

static_assert(
    First::nbElementFirstEnum == Second::nbElementSecondEnum,
    "Not the same number of element in the enums.");
/*static_assert(  
    First::nbElementFirstEnum == Second::nbElementSecondEnum, 
    "Not the same number of element in the enums." + First::nbElementFirstEnum + " " + Second::nbElementSecondEnum);*/

但我希望能够在断言消息中打印 First::nbElementFirstEnum 和 Second::nbElementSecondEnum 的值(就像在注释版本中显然不起作用).我尝试使用带有#"的宏连接.我还尝试使用可变参数模板，使用 %10 检索每个数字并将0"字符添加到检索到的值中，但我得到的只是一个 constexpr char[].

But I would like to be able to print the value of First::nbElementFirstEnum and Second::nbElementSecondEnum in the assert message (like in the commented version which obviously doesn't work). I have tryed using macro concatenation with "#". I also tryed using variadic templates, retrieveing with %10 each number and adding the '0' character to the value retrieved, but all I get is a constexpr char[].

所以我的问题是如何让我的枚举值打印在字符串文字中.

So my question is how can I get my enums values to be printed in a string literal.

可能的重复:

C++11 static_assert:参数化错误消息

在 static_assert 输出中集成类型名称?

最有趣的话题是这个:在编译时打印 sizeof(T)但我不想通过警告或注释代码来了解这些值.

The most interesting topic was this one: Printing sizeof(T) at compile time But I don't want to have a warning, or decomment code to know the values.

推荐答案

这基本上是可行的，尽管稍加努力也可以打破(通过使 V1 和 V2 之和为 256 的倍数).所以，我认为您的解决方案更丑陋但仍然更强大.

This basically works, although it's possible to break with a little effort (by making V1 and V2 sum to a multiple of 256). So, I think your solution is uglier but still more robust.

template <int V1, int V2> struct AssertEquality
{
    static const char not_equal_warning = V1 + V2 + 256;
};

template <int V> struct AssertEquality<V, V>
{
    static const bool not_equal_warning = 0;
};

#define ASSERT_EQUALITY(V1, V2) static_assert( 
    AssertEquality<static_cast<int>(V1), 
                   static_cast<int>(V2)>::not_equal_warning == 0, 
    #V1 " != " #V2 );

// ...

ASSERT_EQUALITY(First::nbElementFirstEnum, Second::nbElementSecondEnum);

输出看起来像:

g++ -std=c++0x -c chksz.cpp
chksz.cpp: In instantiation of ‘const char AssertEquality<3, 2>::not_equal_warning’:
chksz.cpp:40:124:   instantiated from here
chksz.cpp:5:53: warning: overflow in implicit constant conversion
chksz.cpp:40:1: error: static assertion failed: "First::nbElementFirstEnum != Second::nbElementSecondEnum"

<小时>

作为参考，这个原始版本依赖于 gcc 打印 static_assert 消息，即使布尔条件根本无法编译.

---

For reference, this original version depended on gcc printing the static_assert message even when the boolean condition doesn't compile at all.

template <typename Enum1, int Max1, typename Enum2, int Max2>
struct AssertSameSizeEnums;

template <typename Enum1, int EnumMax, typename Enum2>
struct AssertSameSizeEnums<Enum1, EnumMax, Enum2, EnumMax> {};
// only define the special case where Max1 and Max2 have the same integer value

#define ASSERT_SAME_SIZE_ENUMS(E1, M1, E2, M2) static_assert( 
    sizeof(AssertSameSizeEnums<E1, E1::M1, E2, E2::M2>), 
    #E1 "::" #M1 " != " #E2 "::" #M2 );

enum class First {
    a, b, c, nbElementFirstEnum,
};
enum class Second {
    a, b, c, nbElementSecondEnum,
};

ASSERT_SAME_SIZE_ENUMS(First, nbElementFirstEnum, Second, nbElementSecondEnum);

请注意，我将您的枚举更改为强类型，否则枚举的常量名称会发生​​冲突.如果你有弱类型枚举，传递给宏的 First 和 Second 应该命名封闭范围.

Note I changed your enums to be strongly-typed, because otherwise the enumerated constant names clashed. If you have weakly-typed enums, the First and Second passed to the macro should name the enclosing scope.

现在，如果我注释掉其中一个值(因此枚举的大小不同)，我会得到:

Now, if I comment out one of the values (so the enums are different sizes), I get:

g++ -std=c++0x -c chksz.cpp
chksz.cpp:25:113: error: invalid application of ‘sizeof’ to incomplete type ‘AssertSameSizeEnums<First, 3, Second, 2>’
chksz.cpp:25:1: error: static assertion failed: "First::nbElementFirstEnum != Second::nbElementSecondEnum"

看看整数值如何在不完整类型错误中显示，以及在静态断言中的符号名称?

See how the integer values are displayed in the incomplete type error, and the symbolic names in the static assertion?

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