空指针作为 C++ 中的模板参数 [英] Void pointer as template argument in C++

问题描述

以下内容无法编译:

template<void *p>
class X {
// ...
};

int r;

int main()
{
    X<&r> x;

    return 0;
}

错误信息是

x.cc:10:6: 错误:无法转换模板参数‘&r' 到 'void*'

x.cc:10:6: error: could not convert template argument ‘& r’ to ‘void*’

将 &r 显式转换为 (void *) 也无济于事.错误信息变为:

Explicitly casting &r to (void *) doesn't help either. The error message becomes:

x.cc:10:14: 错误:无法将模板参数‘(void*)(& r)’转换为‘void*’

x.cc:10:14: error: could not convert template argument ‘(void*)(& r)’ to ‘void*’

标准的哪一部分规定了这种行为?GCC 版本为 gcc version 5.2.1 20151003 (Ubuntu 5.2.1-21ubuntu2)

Which part of the standard specifies that behaviour? The GCC version is gcc version 5.2.1 20151003 (Ubuntu 5.2.1-21ubuntu2)

请注意，使用例如int * 而不是 void * 按预期工作.

Please note that using e.g. int * instead of void * works as expected.

(回答我自己)

当指定 -std=c++1z 时，它不与 gcc HEAD 6.0.0 20151016(实验性)一起工作，既不隐式也不显式转换为空*".

It does not work with gcc HEAD 6.0.0 20151016 (experimental) when specifying -std=c++1z, neither with implicit nor with explicit casting to "void *".

它确实与 clang HEAD 3.8.0 (trunk 250513) 一起使用，并且从(至少)clang 3.6.0 (tags/RELEASE_360/final) 指定 --std=c++1z 并显式转换为 *void *" 时.如果没有明确的演员表，clang 会抱怨如下:

It does work with clang HEAD 3.8.0 (trunk 250513) and has been since (at least) clang 3.6.0 (tags/RELEASE_360/final) when specifying --std=c++1z and explicitly casting to *void *". Without the explicit cast, clang complains as follows:

x.cc:10:7: 错误:在转换后的常量表达式中不允许从int *"到void *"的转换

x.cc:10:7: error: conversion from 'int *' to 'void *' is not allowed in a converted constant expression

负责在 c++ 语言规范中修复此错误的是 N4268 已经实现了.

Responsible for fixing this bug in the c++ language specification is N4268 which clang already implements.

推荐答案

我不能引用你头顶的章节和诗句(欢迎编辑)，但你试图做的事情不允许在C++.

I can't quote you the chapter and verse off the top of my head (edits are welcome), but what you are trying to do is not allowed in c++.

模板参数在编译时必须是已知的.指针仅在链接时解析，除非:

Template parameters must be known at compile time. Pointers are only resolved at link time unless:

1. 它们在模板参数列表中默认使用 = nullptr.

它们是成员函数指针(在编译时是已知的，因为它们只是偏移量).

they are member function pointers (which are known at compile time since they are merely offsets).

例如，这将编译:

template<void * = nullptr>
class X {
    // ...
};

int r;

int main()
{
    X<nullptr> x;

    return 0;
}

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