基于传递给构造函数的参数数量的推导指南

Question

这是我正在尝试但似乎不起作用的方法：我想根据 class 对象的实例化方式来切换编译时开关。如果只有一个构造函数参数，LengthOpt 应等于 false，否则等于 true（我的实现有更多构造函数，其中开关应默认为 true.

我试图创建一个推导指南，但如果模板参数没有显示为构造函数参数（这是一个真正的失败者），显然它不起作用。有谁知道这个问题的不太详细的解决方案吗？

Code:

#include <cstring>
#include <iostream>

const char* str = "some input string";


template <bool LengthOpt>
class some_class
{
public:
    some_class(const char*) {
        std::cout << std::boolalpha << LengthOpt << std::endl;
    }
    some_class(const char*, size_t len) {
        std::cout << std::boolalpha << LengthOpt << std::endl;
    }
};

template <bool LengthOpt> some_class(const char*) -> some_class<false>;
template <bool LengthOpt> some_class(const char*, size_t) -> some_class<true>;

int main()
{
    some_class A(str);
    some_class B(str, strlen(str));
}

错误:

<source>:19:27: error: deduction guide template contains a template parameter that cannot be deduced
template <bool LengthOpt> some_class(const char*) -> some_class<false>;
                          ^
<source>:19:16: note: non-deducible template parameter 'LengthOpt'
template <bool LengthOpt> some_class(const char*) -> some_class<false>;
               ^
<source>:20:27: error: deduction guide template contains a template parameter that cannot be deduced
template <bool LengthOpt> some_class(const char*, size_t) -> some_class<true>;
                          ^
<source>:20:16: note: non-deducible template parameter 'LengthOpt'
template <bool LengthOpt> some_class(const char*, size_t) -> some_class<true>;
               ^
<source>:24:16: error: no viable constructor or deduction guide for deduction of template arguments of 'some_class'
    some_class A(str);
               ^
<source>:11:5: note: candidate template ignored: couldn't infer template argument 'LengthOpt'
    some_class(const char*) {
    ^
<source>:19:27: note: candidate template ignored: couldn't infer template argument 'LengthOpt'
template <bool LengthOpt> some_class(const char*) -> some_class<false>;
                          ^
<source>:8:7: note: candidate template ignored: could not match 'some_class<LengthOpt>' against 'const char *'
class some_class
      ^
<source>:14:5: note: candidate function template not viable: requires 2 arguments, but 1 was provided
    some_class(const char*, size_t len) {
    ^
<source>:20:27: note: candidate function template not viable: requires 2 arguments, but 1 was provided
template <bool LengthOpt> some_class(const char*, size_t) -> some_class<true>;
                          ^
<source>:25:16: error: no viable constructor or deduction guide for deduction of template arguments of 'some_class'
    some_class B(str, strlen(str));
               ^
<source>:14:5: note: candidate template ignored: couldn't infer template argument 'LengthOpt'
    some_class(const char*, size_t len) {
    ^
<source>:20:27: note: candidate template ignored: couldn't infer template argument 'LengthOpt'
template <bool LengthOpt> some_class(const char*, size_t) -> some_class<true>;
                          ^
<source>:11:5: note: candidate function template not viable: requires 1 argument, but 2 were provided
    some_class(const char*) {
    ^
<source>:8:7: note: candidate function template not viable: requires 1 argument, but 2 were provided
class some_class
      ^
<source>:19:27: note: candidate function template not viable: requires 1 argument, but 2 were provided
template <bool LengthOpt> some_class(const char*) -> some_class<false>;
                          ^
4 errors generated.
ASM generation compiler returned: 1
<source>:19:27: error: deduction guide template contains a template parameter that cannot be deduced
template <bool LengthOpt> some_class(const char*) -> some_class<false>;
                          ^
<source>:19:16: note: non-deducible template parameter 'LengthOpt'
template <bool LengthOpt> some_class(const char*) -> some_class<false>;
               ^
<source>:20:27: error: deduction guide template contains a template parameter that cannot be deduced
template <bool LengthOpt> some_class(const char*, size_t) -> some_class<true>;
                          ^
<source>:20:16: note: non-deducible template parameter 'LengthOpt'
template <bool LengthOpt> some_class(const char*, size_t) -> some_class<true>;
               ^
<source>:24:16: error: no viable constructor or deduction guide for deduction of template arguments of 'some_class'
    some_class A(str);
               ^
<source>:11:5: note: candidate template ignored: couldn't infer template argument 'LengthOpt'
    some_class(const char*) {
    ^
<source>:19:27: note: candidate template ignored: couldn't infer template argument 'LengthOpt'
template <bool LengthOpt> some_class(const char*) -> some_class<false>;
                          ^
<source>:8:7: note: candidate template ignored: could not match 'some_class<LengthOpt>' against 'const char *'
class some_class
      ^
<source>:14:5: note: candidate function template not viable: requires 2 arguments, but 1 was provided
    some_class(const char*, size_t len) {
    ^
<source>:20:27: note: candidate function template not viable: requires 2 arguments, but 1 was provided
template <bool LengthOpt> some_class(const char*, size_t) -> some_class<true>;
                          ^
<source>:25:16: error: no viable constructor or deduction guide for deduction of template arguments of 'some_class'
    some_class B(str, strlen(str));
               ^
<source>:14:5: note: candidate template ignored: couldn't infer template argument 'LengthOpt'
    some_class(const char*, size_t len) {
    ^
<source>:20:27: note: candidate template ignored: couldn't infer template argument 'LengthOpt'
template <bool LengthOpt> some_class(const char*, size_t) -> some_class<true>;
                          ^
<source>:11:5: note: candidate function template not viable: requires 1 argument, but 2 were provided
    some_class(const char*) {
    ^
<source>:8:7: note: candidate function template not viable: requires 1 argument, but 2 were provided
class some_class
      ^
<source>:19:27: note: candidate function template not viable: requires 1 argument, but 2 were provided
template <bool LengthOpt> some_class(const char*) -> some_class<false>;
                          ^
4 errors generated.
Execution build compiler returned: 1

Answer 1

您不需要模板参数，只需执行以下操作：

some_class(const char *) -> some_class<false>;
some_class(const char *, size_t) -> some_class<true>;

演绎指南就像函数，但它们的所有模板参数都必须可以从它们的函数参数中推导出来。

template <bool LengthOpt> some_class(const char*) -> ... 变成类似 template <bool LengthOpt> void foo(const char*) 的东西，其中 LengthOpt 是 non-deducible.

基于传递给构造函数的参数数量的推导指南

Deduction guide based on number of parameters passed to constructor

c++

constructor-overloading

c++17

ctad

deduction-guide