StringBuilder如何决定它的容量应该有多大？

发布于 2024-10-08 17:38:18 字数 243 浏览 0 评论 0原文

我知道当 sb 已满时使用 sb.Append(..) 时，StringBuilder 对象会分配更多内存。但容量会增加多少呢？

    StringBuilder sb = new StringBuilder(5);
    sb.Append("0123456789");

现在某人的能力是多少，为什么？什么是乘数？

只是为了清楚起见。我问的是容量而不是长度。

谢谢！

原文

I know that the StringBuilder object allocates more memory when you use sb.Append(..) when the sb is already at capacity. But how much does that capacity increase?

    StringBuilder sb = new StringBuilder(5);
    sb.Append("0123456789");

Now what is the capacity of sb and why? What is the multiplier?

Just for clarity. I am asking about capacity and not length.

Thanks!

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扮仙女 2024-10-15 17:38:18

除某些特殊情况外，容量每次都会加倍：

如果加倍还不够，则容量会进一步增加到所需的确切数量。
有一个上限 - 0x7fffffff。

您可以使用.NET Reflector或下载参考源来查看该算法。

我无法发布官方 .NET 实现的源代码，但这里是 Mono 实现的代码：

// Try double buffer, if that doesn't work, set the length as capacity
if (size > capacity) {

    // The first time a string is appended, we just set _cached_str
    // and _str to it. This allows us to do some optimizations.
    // Below, we take this into account.
    if ((object) _cached_str == (object) _str && capacity < constDefaultCapacity)
        capacity = constDefaultCapacity;

    capacity = capacity << 1;  // This means "capacity *= 2;"

    if (size > capacity)
        capacity = size;

    if (capacity >= Int32.MaxValue || capacity < 0)
        capacity = Int32.MaxValue;

    if (capacity > _maxCapacity && size <= _maxCapacity)
        capacity = _maxCapacity;
}

我还建议您不要编写依赖于此特定算法的代码，因为它是实现细节，而不是其他东西这是由接口保证的。

The capacity doubles each time apart from some special cases:

If doubling is not enough then the capacity is further increased to the exact amount that is required.
There is an upper limit - 0x7fffffff.

You can see the algorithm by using .NET Reflector or downloading the reference source.

I can't post the source code for the official .NET implementation but here's the code for the Mono implementation:

// Try double buffer, if that doesn't work, set the length as capacity
if (size > capacity) {

    // The first time a string is appended, we just set _cached_str
    // and _str to it. This allows us to do some optimizations.
    // Below, we take this into account.
    if ((object) _cached_str == (object) _str && capacity < constDefaultCapacity)
        capacity = constDefaultCapacity;

    capacity = capacity << 1;  // This means "capacity *= 2;"

    if (size > capacity)
        capacity = size;

    if (capacity >= Int32.MaxValue || capacity < 0)
        capacity = Int32.MaxValue;

    if (capacity > _maxCapacity && size <= _maxCapacity)
        capacity = _maxCapacity;
}

I would also recommend that you don't write code that relies on this specific algorithm as it is an implementation detail, and not something that is guaranteed by the interface.

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