C++核心准则是否禁止（或强烈不鼓励）包含std::string成员的类拥有默认构造函数？

Great question! Let's break this down clearly: the C++ Core Guidelines do not prohibit classes with std::string members from having default constructors, but they strongly discourage default constructors that can throw exceptions. This is exactly what MSVC's strict code analysis is flagging with warnings C26455 and C26447.

Why the Warnings Pop Up #

The core rule here is F.6: Make default constructors noexcept from the guidelines. The logic is simple: if a default constructor can throw, operations that depend on it (like default-initializing a container of your class, or performing move/swap) can fail unexpectedly. Move and swap are intended to be "no-fail" operations for reliability, so a throwing default constructor undermines that guarantee.

Your example hits a classic catch-22:

• Without noexcept, you get C26455, since std::string("Anonymous") requires heap allocation (which can fail, making the constructor throwable).
• Adding noexcept triggers C26447, because in C++17, the std::string constructor you're using still carries a theoretical exception risk (even if small strings rarely trigger allocation failures).

Initializing the std::string inline (e.g., std::string my_name{ "Anonymous" };) doesn't fix this, as the compiler treats that initialization as part of the default constructor's logic. Only relying on std::string's default empty constructor avoids warnings, since that operation is noexcept in C++17 and later.

Practical Solutions for C++17 #

Here are straightforward ways to resolve the warnings without overcomplicating your code:

1. Use std::string's Default Empty Constructor #

If your use case allows a default Person to have an empty name (instead of "Anonymous"), this is the cleanest, guideline-compliant fix:

#include <string>
class Person {
private:
    std::string my_name; // Default-constructed to empty string (noexcept)
public:
    Person() noexcept = default; // No warnings!
};

2. Delay String Initialization #

If you need the "Anonymous" default but can accept setting it later, refactor to defer the heap allocation:

#include <string>
class Person {
private:
    std::string my_name;
    bool is_initialized = false;
public:
    Person() noexcept = default;

    // Throwing constructor is allowed per guidelines
    explicit Person(std::string name) : my_name(std::move(name)), is_initialized(true) {}

    // Setter can throw (also permitted)
    void set_name(std::string name) noexcept(false) {
        my_name = std::move(name);
        is_initialized = true;
    }
};

This keeps your default constructor noexcept while still supporting the "Anonymous" value when needed.

3. Suppress Warnings (Last Resort) #

If you must initialize the string in the default constructor and can't change your design, suppress the specific warnings with a clear comment explaining the tradeoff:

#include <string>
class Person {
private:
    std::string my_name;
public:
    // Suppress warnings: Accept minimal allocation failure risk for required default value
    #pragma warning(suppress: 26455 26447)
    Person() : my_name{ "Anonymous" } {}
};

This is a compromise, but better than disabling warnings globally. Only do this if you're confident the allocation failure risk is negligible for your use case.

Why C++20 Fixes the Warning #

The warning disappears in C++20 thanks to Small String Optimization (SSO) and improved compiler analysis. Most standard library implementations (including MSVC's) store small strings directly in the std::string object (no heap allocation) if they fit in a fixed buffer. "Anonymous" (9 characters) easily fits in this buffer.

In C++20, MSVC's compiler can detect that the string literal will trigger SSO, meaning no heap allocation occurs during initialization. Since no allocation is needed, the std::string constructor becomes effectively noexcept here, so both warnings are resolved. The constexpr ""s operator isn't the main factor—this is about the compiler confirming no allocation will happen.

Should You Ignore the Guidelines? #

No, if you can avoid it. The guidelines exist to boost code reliability; throwing default constructors can lead to subtle bugs (like hard-to-debug container initialization failures). But if your use case truly requires the default constructor to initialize the string, suppressing the warnings (with context) is a reasonable compromise.

内容的提问来源于stack exchange，提问作者Adrian Mole