C++ 模板发送“虚假参数”是一种不好的做法吗？

Question

std::stoi is throwing some errors in specific cases. I don't want to use try/catch block, so I googled a little bit about char convertion and saw that std::from_chars was doing exactly what I wanted, without those try/catch block.

Obviously, using std::from_chars directly works pretty well, but I don't like the syntax. So I started writing my own ToInt() , ToFloat() , etc. And was like: "ok, that sounds stupid, let's use template".

Here I am:

#include <string>
#include <charconv>
#include <iostream>

template <typename T>
T ToNumber(const char* str, T varType)
{
    if (!str)
        return 0;
    T var = 0;
    std::from_chars(str, str + strlen(str), var);
    return var;
}

int main()
{
    std::string t = "123.5";

    auto a = ToNumber(t.c_str(), (int)0); // a will be an int (123)
    float b = ToNumber(t.c_str(), (int)0); // b will store the value as int (123)
    auto c = ToNumber(t.c_str(), (float)0); // c will be a float (123.5)

    std::cout << a;
    return 0;
}

It actually works, that is not the problem. But I was wondering... Is it a good practice to send a "false argument" ( T varType ) in order to get the cast done and enable auto ?

If not, what is the clever way to write such a function?

Thanks in advance

Answer 1

You are not checking the return value of std::from_chars for failure, eg:

auto [p, ec] = std::from_chars(str, str + strlen(str), var);
if (ec != std::errc()) {
    ...
}

That said, another way to deal with the template argument T without using a type-casted input parameter is to simply specify the desired type explicitly at the call site instead, eg:

template <typename T>
T ToNumber(const char* str)
{
    if (!str)
        return T{};
    T var{};
    auto [p, ec] = std::from_chars(str, str + strlen(str), var);
    if (ec != std::errc())
        return T{};
    return var;
}

auto a = ToNumber<int>(t.c_str());
float b = ToNumber<int>(t.c_str());
auto c = ToNumber<float>(t.c_str());

Otherwise, you can use template argument deduction via a reference output parameter (just like std::from_chars() does), eg:

template <typename T>
bool ToNumber(const char* str, T &var)
{
    if (!str)
        return false;
    auto [p, ec] = std::from_chars(str, str + strlen(str), var);
    return (ec == std::errc());
}

int a;
ToNumber(t.c_str(), a);

int tmp;
ToNumber(t.c_str(), tmp);
float b = tmp;

float c;
ToNumber(t.c_str(), c);