How to Safely Pass a C++ String View as Input to a C-interface API

Last time, we saw that passing a C++ std::[w]string_view to a C-interface API (like Win32 APIs) expecting a C-style null-terminated string pointer can cause subtle bugs, as there is a requirement impedance mismatch. In fact:

• The C-interface API (e.g. Win32 SetWindowText) expects a null-terminated string pointer
• The STL string views do not guarantee null-termination

So, supposing that you have a C++17 (or newer) code base that heavily uses string views, when you need to interface those with Win32 API function calls, or whatever C-interface API, expecting C-style null-terminated strings, how can you safely pass instances of string views as input parameter?

Invoking the string_view/wstring_view’s data method would be dangerous and source of subtle bugs, as the data returned pointer is not guaranteed to point to a null-terminated string.

Instead, you can use a std::string/wstring object as a bridge between the string views and the C-interface API. In fact, the std::string/wstring’s c_str method does guarantee that the returned pointer points to a null-terminated string. So it’s safe to pass the pointer returned by std::[w]string::c_str to a C-interface API function that expects a null-terminated C-style string pointer (like PCWSTR/LPCWSTR parameters in the Win32 realm).

For example:

// sv is a std::wstring_view

// C++ STL strings can be easily initialized from string views
std::wstring str{ sv };

// Pass the intermediate wstring object to a Win32 API,
// or whatever C-interface API expecting 
// a C-style *null-terminated* string pointer.
DoSomething( 
    // PCWSTR/LPCWSTR/const wchar_t* parameter
    str.c_str(), // wstring::c_str

    // Other parameters ...    
);

// Or use a temporary string object to wrap the string view 
// at the call site:
DoSomething(
    // PCWSTR/LPCWSTR/const wchar_t* parameter
    std::wstring{ sv }.c_str(),

    // Other parameters ...
);