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c++exceptionvectorlibstdc++
MarkB
MarkB

Reputation: 1988

Why does std::vector::shrink_to_fit() have no effect when exceptions are disabled?

When compiling using gcc 14.2 (or Clang), std::vector::shrink_to_fit() has no effect when exceptions are disabled. This can be demonstrated through the following code (Live on Compiler Explorer):

#include <vector>
#include <iostream>

int main()
{
#ifdef __cpp_exceptions
    std::cout << "with exceptions" << '\n';
#endif

    auto v = std::vector<int>(100);
    v.resize(42);
    v.shrink_to_fit();
    std::cout << v.capacity() << '\n';  // prints 100 when exceptions are disabled, 42 otherwise
    {
        auto v2 = v;
        v.swap(v2);
    }
    std::cout << v.capacity() << '\n';  // prints 42
    return 0;
}

Although shrink_to_fit() is non-binding, I would not have expected disabling exceptions to cause this difference in behavior. What is the rationale for this library implementation choice?

Bonus question: Does -fno-exceptions cause other subtle/unexpected libstdc++ differences that can potentially impact program performance?

Upvotes: 2

Views: 136

Answers (0)

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