基于传递给构造函数的参数数的推导

问：

这是我正在尝试的似乎不起作用的东西：我想根据类对象的实例化方式切换编译时开关。如果只有一个构造函数参数，则应等于 ，否则为 （我的实现具有更多构造函数，其中开关应默认为 .LengthOptfalsetruetrue

我试图创建一个演绎指南，但显然，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），它就不起作用。有谁知道这个问题不太冗长的解决方案？

代码：

#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